Located somewhere in the French Riviera is the Forest of Digne. It was the midnight-meeting place for International Rescue's London agent Lady Penelope Creighton-Ward and MI.5 agent Jimmy Bondson of the British Secret Service.
He tells her that plans of a nuclear device have been stolen from Captain Blacker's yacht - after Blacker himself had been murdered, and the yacht blown up in the harbor by a concealed bomb. Such is the urgency to find and recover those plans, that he's called International Rescue to ask for the secret organisation's help in doing so.
The unusual alliance (including not only Penelope herself, but Jeff Tracy, Scott Tracy, Virgil Tracy and Gordon Tracy, and Thunderbirds 1, 2 and 4) ultimately pays off, though; for when Bondson is again summed to a late-night meeting in the forest, he is directed to a certain large tree - where the now-recovered plans are waiting for him.
