Warren Grafton visits Lady Penelope, wanting her to invest in his monorail project. She declines, and finds out from Parker that Grafton's chauffeur is a crook. She instead tells Grafton she'll contact Jeff Tracy, believing that he would be interested in the monorail. Grafton leaves, but tells his chauffeur Malloy that he has disabled the mansion's alarms and knows where the safe is. The two return to New York, where Grafton calls a meeting with his other investors. Via illegal methods they've raised more than half the money, and Grafton plans to get the rest via two methods: getting Jeff Tracy to invest, and stealing Penelope's jewel collection.

Jeff, Brains and Tin-Tin go to New York, and ride on the new monorail. Grafton tries to hype it up, and the entire thing is automatically run -there are no staff onboard at all. The monorail works via automatic signals, and in case of an emergency, Grafton has a fleet of helijets patrolling the track to report any problems. Helijet 304, piloted by a guy called Joe, is on patrol, but his helijet is struck by lightning and crashes into part of the track. Joe ejects in time, and notices that the automatic signals have been broken. He tries to radio his base to get them to stop the train.

Grafton shows everyone the power unit, and Brains and Tin-Tin examine it. Meanwhile, another helijet pilot called Stan sees Joe, and saves him. They see that the track has been badly damaged, and to make it worse, it's on a bridge, which is falling apart. With no way to manually stop the train, they think the passengers on it are finished. Meanwhile, Malloy and Selsden break into Penelope's home, and quickly find where the safe is. Selsden attempts to open it.

Brains and Tin-Tin open up the power unit to try and find a away to stop the train, but are unsuccessful. Jeff suggests to Grafton that they contact International Rescue. Meanwhile, Selsden is able to open the safe, but it has an alarm on it that's on a different system to the others, alerting Penelope and Parker that they have visitors. Malloy and Selsden take the jewels and attempt to flee in their car, but they are shot at by Penelope and Parker.

With their own car damaged, the two crooks steal FAB 1. But before they can leave, Penelope activates a remote control, causing FAB 1 to keep driving around in circles. Back on the monotrain, Brains tries to stop the train, and this time, the brakes on it activate. The train slows down just in time, but comes to a stop on the damaged bridge, which is still falling apart. Scott and Virgil arrive in Thunderbirds 1 and 2.

Virgil uses Thunderbird 2's grabs to remove the carriage everyone is in, and he moves it to safety moments before the bridge collapses, destroying it along with the rest of the train. Jeff believes Grafton can look forward to being behind bars, and whilst Grafton believes he can get away with it, the opposite happens. He's soon jailed along with the rest of his associates, and whilst he thinks he can still turn things around, everyone tells him he's wasting his time. He says that time is one thing they've got plenty of.

Notes

-This episode is another one than was extended from 30 to 50 minutes. It's believe the added scenes were the opening car chase along with Malloy and Selsden's attempt to steal Penelope's jewel collection.

-Why Penelope is being followed by two guys at the start of the episode is never explained.

-In Vault of Death is was revealed that Penelope could barely drive, but she's clearly been having lessons since then, as she has no problems driving a car in this episode.

Errors

-Grafton initially calls his organization the "Atlantic Pacific Monorail Company", but later on "Company" gets swapped out for "Corporation".
-Would it really be cheaper for Grafton to have a helijet fleet as opposed to hiring a few people to serve as staff on the monotrain?
-When Joe sits down in Stan's helijet, look at the bottom of his chair -a human hand comes into view and holds onto the chair.
-When Selsden tells Malloy it should only take him about a minute to open the safe, Malloy's torch starts to smoke -the bulb has become so hot it is setting the case alight.
-During this shot, watch the statue -at one point it wobbles for a second or so.
-When the train stops, there is an explosion in front of it for seemingly no reason. It appears the explosion was mistimed slightly, and went off a few seconds before it should have done.