International Rescue agree to help the British Secret Service recover some stolen plans for a nuclear device, but Lady Penelope's plan to trap the thieves goes horribly wrong...

A small harbour town on the French Riviera at night. A diver named Carl emerges from the water and sneaks on board a boat. He shoots a crewmember woking at a desk and hurriedly searches through the drawers. He finds a can containing some papers and puts it inside his wetsuit. Back in the water he clamps a small device to the boat. Another boat approaches containing two British secret agents. Before they get to the first boat it explodes in front of them. One of them, Bondson elects to dive in; he too is after the papers. In the cabin of the wreck, Bondson finds the body and disovers that the papers are missing.

At Tracy Island, Brains is experimenting with a model submarine in the swimming when Alan jumps in. John radios in from Thunderbird 5 to say that Bondson has asked for International Rescue's help; if not, the world could be destroyed. Jeff elects to send one of their agents.

Lady Penelope arrives in France in Fireflash and is met by Parker in Fab 1. Faking a French accent, she calls Bondson and tells him to meet her in a clearing a forest at midnight. Once there, Penelope holds him at gunpoint and never shows her face. Bondson explains that the papers were plans of a nuclear device and he was to collect them the previous night. Unfortunately they have fallen into the wrong hands. Penelope leaves without telling him if they will help or not.

The next morning, posing as Gayle Williams, Penelope has bought a yacht. She tells Parker to make it well known that 'Miss Williams' intends to find whoever commited the crime. Later Carl reads a newspaper and learns this. He goes off in a boat to a boathouse and dons his diving gear. He enters a submarine underwater and meets two of his cronies. He shows them the newspaper and tells them he intends to kill Williams.

That night, Penelope sends Parker away to the casino. She calls Jeff and tells him her 'arrangements' are complete and they should have the plans by midnight. Carl arrives in a boat and holds Penelope at gunpoint. Her flippancy annoys him and he forces her onto the boat. He takes her to his boathouse and explains that he will leave her in the boat with a bomb. When it explodes, she will die and they can use the distraction as cover for their escape in the submarine while the patrol boat investigates the submarine. Penelope is allowed to adjust her face but she can only signal to Jeff as Carl is listening. She manages to convey information about a boathouse, a submarine and a bomb before beging cut off. While she is tied up, the Tracys are baffled by their image from the mirror on the floor. As Penelope talks, she correctly fills in the gaps in what they already know. Jeff sends Scott off in Thunderbird 1 and Virgil and Gordon in Tunderbird 2 with Thunderbird 4 inside the pod.

Carl leaves and tells Penelope she has about an hour left. She tries to talk through the mirror which is on the other side if the room but Jeff cannot hear. The crooks prepare to leave. Penelope shuffles her chair in front of the mirror and tells Jeff to get Thunderbird 1 to track the submarine.

Thunderbird 1 arrives at the coast and Scott attempts to use his sonar scan to detect the submarine. He tells Virgil where it is and Thunderbird 4 is launched. Time begins to run out with the patrol boat on its way. With seconds to go, Thunderbird 4 drills through the hull of the submarine and a transquillizer gas puts the crooks out of action before they can use the bomb. Scott rescues Penelope in Thunderbird 1. Gordon swims into the submarine and locates the plans.

The next night Bondson attends the clearing and Penelope's voice tells him to go to a tree where she has left the plans. Later in Fab 1 Parker confesses that at the casino he gambled away Penelope's yacht.

Verdict: No prizes for guessing which successful franchise this episode was trying to emulate. Instead of achieving its aims it simply sinks to the level of a less-than average Thunderbirds. The villain's evil scheme is ridiculously overelaborate.

DVD availability: On Volume 5 and the Complete Box Set.

VHS availability: On the Pod 2 Box Set and the Complete Box Set.