In Spain, a frogman shoots the captain of a boat dead, takes something disguised as a battery in a torch and then blows up the ship, hoping to cover his tracks. Meanwhile, two MI5 agents, Bonson and Tetland are about to rendezvous with the boat when it blows up. Bonson searches the wreck and discovers that the captain, Blacker, was murdered and that the 'papers' have been taken from the torch they were kept in.

International Rescue receive a call from Bonson, who tells them that the whole world is in danger if these plans fall into the wrong hands. Jeff sends Lady Penelope to meet with Bonson at midnight in the Forest of Dean. She holds him at gunpoint, and he tells her that the papers are plans for a nuclear device and that Blacker was shot by an unusual weapon.

Lady Penelope heads out for Spain in her yacht and poses as Gail Williams, a model intent on tracking down the saboteurs.

In a submarine the frogman, Karl, reads all about it in his daily newspaper and plans to kill Gail that night.

Penelope gives Parker the night off. He plans to go to nearby Monte Carlo and have a cracking night. Penelope lets Jeff know that she is ready, and then catches Parker sneaking off the boat with his safe-cracking equipment.

When Parker has left, Karl comes aboard and kidnaps Penelope. She is very calm about it all. Karl tells her his whole plan, and she asks to fix her face before she dies. Through her compact, she communicates to Jeff her situation. He sends a meaningless signal back to confirm that he has received Penelope's message, and sends out the boys to help her.

Penelope tries to contact Jeff from the open compact when Karl leaves. She can hear him, but he cannot hear her, and she must get nearer whilst tied to a chair. Eventually she can inform Jeff of the full situation.

Scott manages to pinpoint the location of the submarine, and Gordon sets off in Thunderbird 4, whilst Scott goes to help Lady Penelope. Gordon neutralises the submarine by anaethatising the crew with a gas he pumps into the cabin, whilst Scott lectures Penelope about taking risks. Gordon recovers the plans and Penelope is able to deliver them back to Bonson.

On their return home, Penelope forces Parker to reveal how his night at the casino went. He bashfully tells her that he gambled away her yacht!

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