At London Airport, the control staff give Fireflash 3 clearance to go. The Fireflash takes off, and the copilot reports that their current height is 15,000 feet, and are climbing to 150,000. Control gives them permission to leave the London control area, and to report their position as they cross the coast. The Fireflash then goes through the sound barrier.

Norman reports to the International Air Minister that Fireflash 3 has taken off with no mishaps, and crossed the coast a few minutes ago. However, a mayday alarm then goes off. The Fireflash copilot reports their position, and says the aircraft is losing height rapidly. But then the communication cuts out. The control tower puts the airport into a state of emergency.

The Air Sea Rescue is told to commence Operation Seahawk, and four jets take off. Norman contacts the naval rescue headquarters, and asks for all ships nearest to the Fireflash's position be diverted to search the area. Some rescue boats arrive at the position, and start searching.

The jets also start searching, but after some time has passed one of the pilots reports that their search if negative. Lieutenant Burroughs tells Norman that both air and sea are reporting negative searches -Fireflash 3 has just disappeared. Norman orders for Operation Seahawk to be cancelled. Later, a meeting is held with the International Air Minister. The minister says it's not good enough -that aircraft cost not only 5 million pounds, but 600 lives as well. He has a duty to the public, and orders for all Fireflash planes to be grounded immediately.

He then says he wants the whole project tested, both in the lab and on the aircraft. Patterson, an engineer who works at the airport, says they've already carried out exhaustive tests on Fireflash. He is told to recheck the whole works. The minister says no fault is being blamed on anyone, but there is a fault, and that fault must be eliminated. News of Fireflash being grounded soon reaches the press.

At Tracy Island, everyone is watching a news report. Eddie Kerr says that this morning, British plane Fireflash disappeared on its flight to San Francisco. Earlier this year, on the plane's maiden flight, a bomb was fixed to the plane's undercarriage. It was only due to the intervention of International Rescue that the plane and its passengers were saved. But this time there were no survivors, and now all Fireflash aircraft have been grounded pending tests. John wonders if it's sabotage again, but Tin-Tin doubts it, not with the precautions they take now. Jeff wonders how a plane like that could disappear, but it could be from a hundred and one things. As he brings his desk back down, he thinks they should watch these tests pretty closely.

Jeff contacts Alan in Thunderbird 5, and asks him to monitor all transmissions on the Fireflash tests. Alan says he has been checking on the crash, and according to their automatic fixer, the Fireflash crew radioed a wrong position before they disappeared, and were more than 50 miles out. Jeff thinks that's strange, and tells Alan to keep listening. He then contacts Scott, Virgil and Gordon, and tells them all to be on standby. Further tests are being carried out on Fireflash, and it could mean trouble.

At the airport, Patterson is conducting tests on a Fireflash. Norman asks him how it's going, and he says that they've eliminated metal fatigue. He says they're awaiting results of the radiation test, and then sees them being printed off now. As he checks on the results, he tells Norman that all tests show the Fireflash to be completely OK, so it's over to them now.

Norman calls together a group, including two pilots. He tells the pilots they'll fly the same course as Fireflash 3. The plane they'll be using has been checked by the ground crew, and is in perfect working order. They are to keep in constant touch with control, and are to fly at 150,000 feet high. He can't emphasize how important it is for them to keep in constant touch with control. The pilots board the Fireflash, and move it down the runway.

Norman tells Burroughs that it'll be his job to maintain continuous contact with Fireflash until she arrives in San Francisco. The pilot says the pre-flight tests are completed, and asks for take off clearance. Burroughs say they're clear to go, so the Fireflash takes off. The pilot says they're at 2,500 feet, and are climbing to 150,000. They soon pass through the sound barrier.

In Thunderbird 5, Alan is tracking the Fireflash on the auto-fixer. But as he hears the pilot give his position, he thinks it's odd -the automatic fixer puts them 20 miles north west to where they've just said. The pilot says that everything seems fine, and they're about 50 miles from the crash area. Norman tells him to take a position check once he reaches the area.

The pilot switches to the automatic flight plan. But the copilot, Bob, notices one of the pumps suddenly drop to zero. He tries the elevator power unit, but it doesn't respond.

The pilot tries to contact control, and says they're having trouble with their EPU. But his message crackles and can't be heard properly. The pilot says they've also got trouble with their gyros, and then gives their position. Norman says this is just as they feared -the same trouble in the same position.

The Fireflash starts to go into a dive. But as it does, someone jumps out of it, and deploys their parachute. Alan reports to Tracy Island that the Fireflash has sent out a mayday. The signal was too weak for London to receive, and he himself can only just hear it. Whilst they've given their position, he thinks they're actually 180 miles north west of it.

Jeff tells him to listen out, and see if he can get another fix. Scott, Gordon and Virgil are at the shooting range, but then Jeff contacts them and says that Fireflash is in difficulties. He tells them to come up so he can fill them in on the details. Meanwhile, the Fireflash continues on its dive, and ends up crashing into the sea.

The Fireflash floats, but the pilot says the emergency exit is jammed. He and Bob can't get it open, as the Fireflash begins to sink. They still can't get it open, and soon the Fireflash completely sinks. As this is going on, another aircraft flies over the sea.

The guy who jumped out of the Fireflash fires a flare gun, which gets the aircraft's attention. It flies over to where he is to pick him up. Back on Tracy Island, Jeff tells Scott to take Thunderbird 1, and establish a base on the coastline nearest the crash area. Then, he needs to scan the area electronically. Jeff then tells Virgil to go and take Gordon and Brains, along with Thunderbird 4.

A little later on Thunderbird 1 blasts off. Thunderbird 2 blasts off moments later. At the coastline, a farmer is raking some hay when he notices something descending upon his farm.

Thunderbird 1 comes in to land, and Scott asks if he can give him a hand with his equipment. Meanwhile Virgil says they're approaching the danger zone. Brains asks Virgil to get London Control to send the circuit diagrams of Fireflash's electronics via radio photograph.

Scott gets his mobile control setup in a barn, and then activates something outside: the scanning antenna. Back in Thunderbird 2, Brains tries to locate where the trouble could be, and thinks he's got it... and then realizes he hasn't got it. Meanwhile Scott says he can't find a thing floating in the whole area.

Brains says he has got it, and says if the gyro shaft sheared, it could have damaged the main hydraulic power supply to the flaps, which would cause the aircraft to crash. Gordon asks if they could have crash landed on the sea. Brains says it's possible, it would depend on the skill of the crew. Virgil thinks if that had happened they would have gotten a distress call from the crew's lifeboat. Brains says the crew could still be trapped in the cabin, as if the hydraulics system jammed, the automatic escape hatches would not operate. The crew could still be alive, trapped in the flight deck at the bottom of the sea!

The Fireflash has sunk to the very bottom. The pilot tries to contact London Control, but there's no response. Bob says it's no good, they can't hear them, not even with the new thorium beam transmitter. Above the sea, Thunderbird 2 clears the coastline. Scott contacts Virgil and says he's done a search of the complete area, and the result was negative.

Virgil says they're launching Thunderbird 4, as they think the crew may be trapped in the aircraft on the seabed. Thunderbird 2 drops pod 4, which hits the water. Gordon then moves Thunderbird 4 out of the pod, and into the sea. Gordon asks Scott for a course to take him to the Fireflash crash position.

Scott tells Gordon to steer 107 degrees, magnetic. Thunderbird 4 reaches the position, and Gordon begins searching for the Fireflash. In the Fireflash, the pilots notice Thunderbird 4. They turn on every light they can inside the cabin, trying to get Gordon's attention.

Gordon sees the light, and finds the Fireflash. He contacts Brains and says the Fireflash is in one piece, but he can't see any signs of life yet. Brains tells Gordon to cut of the engines on the tail-plane with a laser beam, as that will make the Fireflash float to the surface. They can then rescue the crew, but Virgil says only if the crew are still alive. Gordon leaves Thunderbird 4.

The pilots see Gordon. Gordon sees them, and reports to Virgil that they're still alive. He says he'll send them a message on the light type, and does so. His message states he's going to cut off the engines to make the plane float, and they need to flash their torch if they agree.

The pilot does so, so Gordon sends another message, stating that they'll get them out once they're on the surface. He then swims back to Thunderbird 4, and activates the craft's laser cutter. He has Thunderbird 4 take position next to one of the Fireflash's engines, and uses the laser to start cutting through it.

He soon cuts through it, and it drops to the seabed. He then moves Thunderbird 4 over to the other engine, and begins to cut through that. Bob says he can hardly breathe, it's so hot.

Gordon cuts through the other engine, and it also drops down. Seconds later, the Fireflash begins to float up. It resurfaces, so Virgil lets Gordon know about it. Gordon says he'll be coming up alongside her.

Thunderbird 4 then resurfaces, but as it does, something goes wrong inside the cabin -one of the consoles has caught fire! As smoke begins to fill the cabin, Gordon appears at one of the windows.

He starts cutting through the glass with another laser cutter. The console fire gets worse, but the pilot says they can't use the extinguisher, as they can hardly breathe now. Meanwhile, Thunderbird 2 drops down a rescue capsule.

Gordon smashes down the glass, and tells the pilots that as soon as the rescue capsule arrives, they must climb aboard it. He then drops back down into the sea, and swims back to Thunderbird 4. As this is happening, the rescue capsule drops down next to the cabin's broken glass.

Gordon reports that the copilot is aboard the capsule, and the pilot will be onboard in a couple of seconds. Virgil contacts the pilots, and finds out they're both onboard. He tells them to hold tight, as the rescue capsule is retrieved back into Thunderbird 2. Virgil tells Scott he's clearing the area, as the Fireflash look like it's going to go up any second. Gordon also moves Thunderbird 4 away from the Fireflash.

Seconds later, the Fireflash does explode, completely destroying it. Later on, everyone watches a news report on TV. Eddie Kerr says that, once again, International Rescue saved the day. The air ministry has received a report from them which may provide the answer to the technical fault which has bugged the Fireflash aircraft over the last few weeks. Jeff tells everyone he'd like to add his thanks to them. He asks where Scott is. Virgil says he flew in a few minutes ago, his take off from England was delayed for some reason.

Scott then comes back in, and says sorry he's late -he had to milk the cows before he took off. Alan then contacts the island, and says that there's a top level meeting going on about the Fireflash right now. At the meeting, everyone is giving out different ideas to what could be wrong with the Fireflash.

The minister says the fault in the Fireflash has been traced to the hydraulic system. But what causes this fault remains to be discovered. They've developed three different theories, all of which place the trouble in the starboard wing. He proposes that they pursue these three theories, as he can't see what else they can do. At Tracy Island, Jeff says he can't see what else they can do -the cause of the Fireflash disasters can only be found when the aircraft is actually in flight. Virgil and Scott suggest they fly the Fireflash, and have Thunderbird 2 fly with it. Jeff decides to contact London straight away, as he thinks it's time for International Rescue to act.

Norman receives a letter from International Rescue, who want to come and test Fireflash over the same route. Burroughs thinks if anyone can establish the fault, they can, as they're the most experienced pilots in the business. Norman decides to send Captain Hanson on the flight with them, as he knows them from the Fireflash's maiden flight. He then says this whole operation is to be top secret, no one is to fly within 100 miles of the test flight path.

Burroughs asks where the letter was posted from -it may let them know where International Rescue are based. Norman checks the letter, and says it was posted at London Airport. Burroughs thinks International Rescue never miss a trick. Alan contacts Tracy Island and says he's been in touch with the airport. Preparations for Virgil's arrival are already well underway.

Jeff says Virgil will arrive at noon. He doesn't know what form the rescue will take, but Virgil is loaded up with pod 4, the diving escape bell and the laser beam cutter. At the airport, the radar operator says International Rescue are crossing the coast now. Norman orders for the Fireflash to be moved out to one of the runways, so Hanson drives it out of the hangar.

Norman instructs for all personnel aside from Hanson to vacate the aircraft. The airport is then sealed off, as International Rescue arrive. Scott requests permission to land, and Norman tells them they're clear to do so. Virgil lands Thunderbird 2 on the runway, next to the waiting Fireflash.

Jeff tells Alan to organize and maintain constant contact between himself, Thunderbird 2 and the Fireflash. Scott boards the Fireflash, and contacts Virgil and Alan. Alan tells him that he'll be maintaining constant contact throughout the test flight. He then contacts control, and tells them they are ready to start testing Fireflash.

Burroughs gives them clearance to take off. Scott thinks it's swell of Hanson to help them out like this. Hanson says if anyone should be grateful, it's himself. When Fireflash was in trouble on its maiden flight, they saved his life. So it's a real privilege to be able to pay them back this way. The Fireflash then takes off.

Alan says the Fireflash has made a good takeoff, and Virgil has just been given clearance by London Control. At the airport, Thunderbird 2 takes off, and follows the Fireflash. As the Fireflash gets to 150,000 feet, Scott asks Virgil how he's doing. Virgil says he'll soon catch them up. Scott then asks Alan for a position check, and provides their present fix.

Alan says Scott's reading is wrong -they're actually 20 miles north west of that point. Hanson thinks that's some miscalculation. Scott tells Alan to tell Virgil. Hanson contacts London Control, and lets them know they've found a fault in the automatic locator, meaning Fireflash was giving a wrong position. The Fireflash starts to descend.

Hanson's communication begins to crackle, and then stops. Norman says this is the same pattern: first the EPU, then the radio goes. Hanson continues to try and get through to London, but tells Scott the radio is dead, they've lost contact with them. Scott says they haven't lost it altogether, and uses a separate radio to contact Alan and Virgil. He tells them what's happened, and how they'll have to relay messages to London via Alan.

Gordon asks about the EDU standby, but Scott says it's gone too. The Fireflash is taking one colossal crash-dive into the Atlantic Ocean -he can't get the nose of it up. Alan then says he's been through to London, and told them about the EDI and radio failures. They are saying to bail out, and then they'll come and pick them up. Scott says if they do that, they'll never know what the fault in the Fireflash was, and they'd be back to square one.

Scott and Virgil decide to try their scheme. Scott finds out from Hanson that they will hit the water in about 15 minutes. Gordon says he'll be seeing him... he hopes. He goes off and gets ready, and opens up a hatch on Thunderbird 2.

Virgil tells Scott to hold the Fireflash dead steady. This is going to be tricky, but they can do it safely provided they don't get too much turbulence. As Thunderbird 2 moves into position beneath the Fireflash, Gordon tells the Fireflash pilots to open up the hatch. As the hatch opens, something moves.

Gordon thinks he just saw someone, but then thinks he's seeing things. He fires the escape bell up through the opened hatch, connecting a cable between it and Thunderbird 2. He then gets taken up via a moving platform, and enters the Fireflash.

Alan says that Gordon has been winched into the starboard wing of the Fireflash and is checking the place out. Tin-Tin asks how long that'll take, and Alan says it depends on what he finds. Gordon starts looking around, and asks where he should start. Scott tells him the EDU should be situated somewhere near the cylinders. Hanson says they've got less then 4 minutes.

Gordon finds the elevator power unit, and also sees that a cable has been cut on it. He starts telling Scott about what's happened, but then gets shot at. Someone tells Gordon that he's ready for International Rescue. Gordon says he doesn't know what his game is, but there are some pretty vital pieces of equipment around here that they don't want to smash if they can help it.

The guy says the Fireflash is finished, and in just a couple of minutes, it's going to make a mighty big splash in the ocean and disappear like all the others. The two start shooting at each other, but the guy looks at the opened hatch. Gordon tells him not to jump, as his parachute would never open in time. But the guy does so anyway.

Gordon contacts Scott and says there's no time to explain. Scott tells him they've got less than 30 seconds until the plane hits the water, and it's too late to bail out. He asks him if he can fix the EPU. Gordon says he can't remake the joint in time, it'd take too long. So instead, he grabs the cut cable...

...and holds the two pieces together, reconnecting it. In doing this, the power to the Fireflash is restored, and it pulls up seconds before hitting the water. The plane is saved, and heads back to London.

Everyone returns to Tracy Island, where they watch another news report which praises International Rescue. The Fireflash run from London to San Francisco is to be resumed next week. Police authorities have completed their investigations into the sabotaging of the aircraft, and state that the efforts of International Rescue have led to the unmasking of the international gang bent on aircraft espionage. Before the report can continue, power to the TV suddenly cuts out.

Grandma says she and Tin-Tin were just putting the apple pies in the oven, but then the fuse blew. Virgil says Gordon will soon fix that. Scott reminds Gordon of what he said after the Fireflash episode -it was just like fixing a fuse. Gordon says here he goes again!