A presenter says excitement is mounting here at Cape Kennedy, as the most daring rocket launch of all time counts down. Colonel Benson has the launch site cleared, and wants thrust checks started in 15 minutes. Time passes, and Sun Probe's thrusters activate. Benson says that thrust is at 17 million pounds, and increasing.

The presenter says that this new launch technique has been made possible by the use of a revolutionary new fuel, the manufacturing of which is top secret. As more time passes, Benson says the thrust is steady, and has the gantry move away from Sun Probe. In another building, Sir Jeremy Hodge and Professor Borender are watching. Jeremy thinks rocket fuel from water hardly seems possible. Borender says that if anyone could hear him, they'd think he had nothing to do with this new process. Jeremy thinks Borender would have succeeded without him, but Borender says he'd been working on this project for ten years without success before he joined him.

The presenter says that only a few minutes remain before they see the liftoff of Sun Probe, so named because its enormous power will enable it to fly into the sun on full power. A final ten second countdown is started, and Sun Probe blasts off, heading straight into space. The presenter says it was known all along that the risk of failure on this project was at liftoff, but the liftoff was very pretty indeed. He says this success would not be better timed, coming as it does two days before the all important international conference at Paris.

Parker and Penelope are in France. Parker says Paris is 20 kilometers away, and asks where she desires to go once they get there. Penelope says the Café Atalante, as that's where she's meeting Sir Jeremy. She says he seems to be in some kind of trouble. Parker asks if Sir Jeremy is the scientific person who helped to form their organization. Penelope says he is, and he helped Brains get certain components for the Thunderbird machines manufactured secretly in Europe. Parker asks if Sir Jeremy is one of them, and Penelope says he is.

At the café, Penelope meets with Jeremy. He thinks it's most decent of her to come all this way to see an old friend. Penelope says she always obeys the call of friendship, so Jeremy decides to drink to that. Penelope says in Paris, she always drinks Pernod. Jeremy tells the waiter what they want, and as he walks off, he passes a guy who is watching Penelope and Jeremy.

Penelope thinks Jeremy didn't bring her all this way for nothing, and wants to know why they're having this midnight rendezvous. Jeremy asks if anyone saw her come here, and she says apart from Parker, nobody knows she's in Paris. He asks her if she knows of his old friend, Professor Borender. Two nights ago, after the international conference, he saw him off by train to Anderbad. But when the train reached its destination, Borender wasn't on it. The waiter then returns, and puts the drinks on the table.

Penelope is about to drink hers, but before she can, the glass suddenly explodes in her hand. It was Parker who shot it, and he says to beg his pardon, but the drink was drugged. The guy at the other table immediately gets up and makes a quick exit, so Jeremy tries to follow him.

The waiter says he's sorry, and says he had no idea. He says that the gentleman at the table asked for a light, and when his eyes were blinded by the flame, the guy must have put something in her glass then. Jeremy says he's lost the guy, and Parker says he's given them the slip. They check his table, and come across a matchbook. Penelope says she doesn't recognize the crest.

Jeremy looks at it, but he doesn't recognize it either. He suggests they go to the heraldic archives and look it up. Penelope says they can go in the morning, and asks him to escort her back to her hotel, and tell her what this is all about. As Parker drives them to the hotel, Penelope asks Jeremy if he's sure he was Professor Borender travel on that train. Jeremy says he is, he saw him off at the station himself. But the attendant on the train says he's mistaken, and that nobody of that name traveled. Penelope asks if he can think of any reason the Professor would disappear. Jeremy says that for the past year, the two of them have been engaged with experiments with sea water. They've just perfected a method of converting sea water into fuel. It's a very delicate process, which can only be performed with the facilities that he and the professor have devised. In the wrong hands, this process could contaminate the oceans of the world.

Penelope says the consequences to mankind could be pretty terrifying. She says she understands his precautions, but he tells her he doesn't think she does, not completely. He asks her if she remembers last week's Sun Probe launch, and she realizes it must have been launched using the new fuel. Jeremy says it was indeed, and in addition to what he's just mentioned, it's possible the discovery of the fuel could upset the balance of power and start a world war. Penelope says that International Rescue owes him a great debt, and are at his service. He asks that they help him find the professor.

Penelope contacts Tracy Island. She asks for information about Borender and the recent conference at Paris. She and Sir Jeremy will be taking the night train to Anderbad tomorrow. Jeff asks if she thinks she'll need and help, but she doesn't think she will until they get to Anderbad. For now, they just want to make a few investigations on the train. Jeff says he'll send Virgil with Thunderbird 2 and the equipment he thinks she might need, and will meet her at Anderbad. Penelope ends the call, as Parker enters her hotel room. She asks him if he's made the reservations for the night train, and he says he has. Penelope tells him that was good shooting he did earlier.

Back on the island, Virgil heads off to board Thunderbird 2. Jeff tells Tin-Tin another rescue is under way. Tin-Tin thinks the boys are sorry they're not joining Penelope in Paris, but Jeff thinks they'll be time for sightseeing later, as right now they're needed in Anderbad. Meanwhile, Alan and Gordon join Virgil inside Thunderbird 2.

Thunderbird 2 rolls down the runway, and then takes off. Back in Paris, Parker waits outside the heraldic archives building in FAB 1. Inside, Penelope and Jeremy talk to the guy running the place, who looks at the matchbook they'd found. He calls it a most interesting heraldic device, and thinks they've come to the right place. He tells them that if they go down to their lower vaults, they'll find what they need.

Penelope and Jeremy head down some steps and enter the vault. They start looking at some of the books, but Penelope comes across one where the cobwebs have been wiped off lately. She opens the book up.

On one of the pages, the crest looks very similar to the one on the matchbook. But as she turns the page, they see that it has been ripped out. Penelope thinks it looks like someone was expecting them, but as soon as she says that, the door at the top of the stairs is suddenly slammed shut.

They can then hear a hissing sound, and realize gas is being pumped into the room. There aren't any windows down here, and Penelope thinks this is a very clever way of disposing with them. Jeremy starts banging on the door, demanding for it to be opened. Penelope thinks that librarian must have been the stranger who tried to poison her, and he must have remembered leaving the matchbook behind.

Penelope contacts Parker and tells him to keep a lookout for a stranger leaving the building, who is probably wearing a green jacket. Parker sees the guy, and says he's just left the building. Parker wants to go and pursuit him.

Penelope tells him to come and get them out of this building first. Parker moves FAB 1 and fires two cables out of it, which hit the back door to the vault. He then moves FAB 1 forwards...

...and the door is ripped off. Parker enters the vault and says he's sorry to keep them waiting. Penelope says the next step is to get to Anderbad. She tells Sir Jeremy to come along, as the two of them have got a train to catch.

Later on, the monotrain departs with Penelope and Jeremy onboard it. They talk to the attendant, Alfred, who is adamant that there was no professor on this train. Jeremy tells him that he saw the professor off at the station. Penelope thinks that perhaps the attendant was mistaken, but he says he wasn't. He then leaves, stating he must go.

Jeremy begins to wonder if he imagined the whole thing. Penelope thinks the attendant is clearly hiding something, but for now, it's cocktail time -no one is going to deprive her of her Pernod this time. Meanwhile, Alfred has gone back to meet with Godber, the guy who was seen earlier. He tells Godber that he told Penelope and Jeremy nothing. Godber asks if he's telling the truth, and Alfred says he has to believe him. Godber thinks Penelope and Jeremy are interfering fools who will be sorry they set foot on the train. They outwitted him twice in Paris, but he'll set a trap that not even Lady Penelope can escape from.

Jeff instructs Virgil on where to land Thunderbird 2. From the landing point, they should be able to get a good view of Penelope's train as it emerges from the Anderbad tunnel. Jeff says its some length of tunnel, and Virgil thinks is must be lonely inside that mountain. Inside the train, Jeremy says he'll have another word with that attendant. Penelope says for now, they should drink their coffee.

But when Penelope picks up her cup, she sees a folded up piece of paper underneath it. She opens the paper up, and it says to beware, they're dealing with desperate men. Later on, Godber decides to deal with Alfred, and hits him over the back of the head with a torch.

He then dumps Alfred off the train, and he rolls down a hill. Penelope contacts Parker in FAB 1, and asks him how he's progressing. Parker says he's keeping on schedule, and saw their train from the summit of the highway ten minutes ago. Penelope says they've interviewed the attendant, but he's too frightened to tell them anything, and now he's disappeared. She says she's retiring for the night, and Parker asks if she has everything she requires. She admits she's missing her usual cup of cocoa, but Parker says he'd slipped a hot flask of her favorite brand into her luggage.

Alfred has survived his fall, and manages to get up. He says he must warn them, he must stop the train! He starts to stumble towards a gate, as Parker continues driving down the road in FAB 1. But Alfred trips over a rock and falls flat on his face, seconds before Parker passes by the gate, missing him.

At Tracy Island, Scott returns in Thunderbird 1. Jeff welcomes him home, and says he's sorry they aren't having a big reception party to meet him after his leave, but they have a major rescue operation in hand. Scott says he understands and hopes he can help too. He then lands Thunderbird 1.

Jeff explains to Scott about what's going on, and how the forces of International Rescue are converging on the city of Anderbad from three directions. Virgil is going to rendezvous with Parker, and Penelope's train is going through the tunnel. Scott asks when Virgil will arrive, and Jeff expects Virgil to call when he's ready. Virgil then contacts them and says everything is going to plan. They should arrive in one hour, and Parker has told him that he expects to arrive ahead of the train. He is going to pick up Penelope at the station. Jeff doesn't think much more can happen before the train gets to Anderbad.

Penelope wakes up and sees that there is someone at her door. She watches the shadow for a few seconds, then picks a gun up from out of her bag. She gets up and opens the door, but there is nobody outside anymore. She then looks down the corridor and sees someone walking away.

She knocks on Jeremy's door and he answers it. She tells him that someone was listening at her door, so they go to investigate. Meanwhile, Godber has now disguised himself as an attendant, and has gone to where the luggage is being stored. He starts looking at Penelope's bags, but before he can open them, Penelope and Godber appear. He asks if he can help them, and Penelope says they're looking for their attendant, who they haven't seen since dinner. Godber explains that he is their new attendant, and will be taking care of them for the second half of the journey. Penelope asks what he wanted with her suitcase.

Godber claims he was merely ascertaining which passengers would be getting off at Anderbad. Alfred was very careless, as he forgot to leave him a passenger list! He then says if there are no more questions, he'll bid them goodnight. As he leaves, Jeremy thinks he's heard that voice before. Penelope goes back to her compartment and contacts Parker. She tells him to leave his monitor switched to telecall, as Sir Jeremy is going to speak to this new attendant. She wants Parker to tell him if this is the man he saw leaving the archives in Paris.

Jeremy starts talking to Godber, and says what he wants to order for breakfast in the morning. As he's doing this, Penelope films Godber via her compact camera, and Parker sees him. As Godber leaves, Parker tells Penelope that was the guy he saw.

Thunderbird 2 lands, and Alan says their father was right -they've got a great view of the railroad and they can see the end of the Anderbad tunnel. Virgil says that's where Penelope's train will be coming from. Gordon then points out that Parker has arrived, and they see Parker stop FAB 1 just in front of Thunderbird 2.

Jeremy says they're nearly as Anderbad, so they should be entering the tunnel soon. The train enters the tunnel moments later. Virgil tells Parker the train shouldn't be long now, so he'd better get down to the station.

Parker starts FAB 1 back up, and leaves. Meanwhile, the train is still making its way through the tunnel. But suddenly it slows down and then stops, as Penelope says there must have been a power failure. But someone shines a light on her and says she's right, there has indeed been a power failure. The voice tells her and Jeremy to come with him.

Alan asks when they're going to get started, but Gordon tells him to relax -they can't do anything until Lady Penelope gets here. Parker then contacts them and says Lady Penelope's train is going to be late, as there has been a power failure in the tunnel. Virgil tells him to find out exactly where that train is, and then report back.

Virgil contacts Tracy Island, and explains about the train having a breakdown in the middle of the tunnel. Jeff tells him to standby with the Monobrake. Tin-Tin thinks Penelope surely would have called in if there had been any trouble. Scott says that tunnel goes straight under the Alpine Belt, so no radio waves can penetrate that far. Virgil's group then see the train leave the tunnel.

Later on, Virgil reports that the train has now come into Anderbad, and Parker has confirmed that Lady Penelope and Sir Jeremy are not on it. Jeff asks if Parker found out where the breakdown took place, and Virgil says it was 17 miles from Anderbad. Jeff tells him to go and find them, so Virgil and Gordon leave Thunderbird 2 in the Monobrake.

Penelope and Jeremy have been taken to a control room. Godber welcomes them to his little subterranean headquarters, and says from here he can control all the traffic in the Anderbad tunnel. His colleague, Roache, has only to switch off the current and the transcontinental rocket is thrown out of action. A useful little trick, but a dangerous one if used too often. Jeremy supposes this was how he kidnapped Professor Borender too. Godber says that when he saw they suspected him, he was tempted to hurl them both from the train, just as he disposed of that stupid attendant. But he thinks they'll be much more useful to him here. Roache then opens a door, revealing Borender.

Godber introduces himself, and whilst he knows they don't know anything about him, he knows much about them and the fascinating discovery they made. Borender says Godber was present at the Paris conference last week, and he knows about their experiments. Godber says that the conversion of sea water into fuel is a brilliant achievement, one he intends to make use of. Borender says they'll never tell him the formula. Godber says he thinks that now he's captured the lovely Lady Penelope, he can make him and Sir Jeremy talk.

The Monobrake enters the tunnel. Virgil asks how far they have to go, and Gordon says 17 miles, so they need to step on it. Virgil asks when the express was due, but Gordon tells him to relax, they've got the place to themselves. Back in the control room, Borender and Jeremy have been tied up. Godber tells them that now they can see he really means business, perhaps they'll tell him the details for their conversion formula. Borender says even if they told him, what good would it do? Godber says a great deal of good, as it'd make him the richest man in the world. He then orders Roache to lower the ladder.

Penelope has been tied to the ladder, which moves downwards and gets in the way of the monorail track. Godber says now all they have to do is wait for the express to pass. He thinks it should be entering the tunnel in about nine minutes. Meanwhile, Virgil and Gordon continue down the tunnel, and see that there are a lot of ventilation shafts around the place.

Jeremy tells Godber that even if they told him the conversion formula, it'd be useless to him without the equipment they've assembled in Paris. Godber says he'll worry about that later. Borender says their experiments are still in the early stages, which is what he said at the conference. One little mistake could pollute the waters of the sea! Jeremy says it could subject the whole of mankind to the most appalling consequences. Godber says enough, and asks Roache how long it is until the train enters the tunnel. Roache questions what Borender say about polluting the waters of the sea, but Godber says to answer his question. Roache says six minutes. Meanwhile, Alan reports that there's still no sign of Virgil or Gordon.

He wants to get onto his hover bike and go into the tunnel, but Jeff says no, he's needed where he is. Scott asks if he can go out in Thunderbird 1, as he can be out there in a couple of hours. Jeff says the express is going to be there in a couple of minutes.

The express train enters the tunnel. Jeremy tells Godber to stop the train, and tells him to cut the power so they can talk the whole thing over. Godber says he has only to tell him their formula, and he shall willingly cut the power.

Gordon tells Virgil to cut the Monobrake's engine. Virgil does so, and they can hear a rumbling in the distance. Virgil says it must be the express, and Gordon asks how far they've come. Virgil says over 16 miles, so they must be around here somewhere. Roache says the train is now four minutes away, and Godber says he will have to soon turn out the lights, so nobody sees them from the train. They may catch a glimpse of Lady Penelope, but by then it will be too late to stop.

Jeremy tells asks him to listen to reason, as the fuel conversion formula would be useless to him. If he cuts Penelope down, they'll explain the whole thing to him in their laboratory in Paris. Outside, Gordon arrives and stays next to the door, just in time to hear Godber say that his plan has failed. Godber decides to leave Lady Penelope to her fate, but Gordon then shoots at him.

Godber takes cover and starts shooting back. As Jeremy and Borender get down, Jeremy asks Roache to stop the train. Roache is about to, but then Godber shoots him in the back before he can hit the button.

Godber then also shoots the control panel, and Borender realizes now they can't stop the train. Godber tells him to get up and move towards him. Borender does so, and Godber tells Gordon he has ten seconds to throw his gun down and come out with his hands up. If he doesn't, he'll throw Borender in front of the express. Outside, Virgil shoots at the cable the ladder is being held by.

This distracts Godber, so Gordon shoots again, blasting the gun out of his hand. Virgil fires again, severing the cables and dropping the ladder out of the way of the monorail track.

Seconds later, the train passes right over Virgil and Penelope. Later on, Alan reports that nothing else has come out of the tunnel other than the express, and that was 20 minutes ago. But he then sees something come out, and sees that the Monobrake has exited the tunnel. He tells his father that it looks like another rescue operation has been successfully completed.

Later on, Alan, Penelope and Jeremy have gone to Café Atalante. Alan isn't impressed with the violin playing, but Penelope thinks he should just drink up his coke. Alan doesn't get it, everyone else got to go to the follies, so why not him? Penelope tells him it's because he's too young. But he should cheer up, as Parker will be along in a minute to take them to the best nightclub in town.

Alan starts complaining again, but then changes his tune when he sees that Parker has brought Tin-Tin along. Alan asks how she got here, and Penelope explains Jeff thought she deserved a vacation. Jeremy says they'd better get going, or they'll miss the floor show. He sees Penelope hasn't touched her Pernod. Penelope says the last time she has a Pernod at this bar, she was just raising it to her lips, when "bang!" -she was off on another adventure. Jeremy tells her to drink up and see what happens this time.

Penelope raises her glass, and there is another bang, followed by another, and another. Up in the sky, a fireworks display has started, something the group enjoys watching.