The lead characters are ex‑astronaut Jeff Tracy, founder of International Rescue, and his five adult sons, who pilot the organisation's primary vehicles: the Thunderbird machines.
[1] To test the Bank of England's vault, Lady Penelope and Parker (voiced by Sylvia Anderson and David Graham) are challenged to break into it.
It soon becomes clear that an absent-minded, workaholic employee called Lambert is still at his desk inside the vault, unaware that he is trapped and running out of air.
At Penelope's suggestion, they set off for London in FAB 1, but Parker deliberately prolongs the journey – driving slowly, taking wrong turns, and then stopping and pretending to be lost.
When Penelope demands an explanation for his behaviour, Parker tells her that when he was in prison he shared a cell with Fred, who said his life's ambition was to rob the Bank of England.
Penelope takes the wheel but proves to be a terrible driver, narrowly avoiding an accident before leaving the road and cutting across country.
However, on arriving in London, the brothers establish that the city's underground power cables prevent them from using Thunderbird 2's Mole pod vehicle to tunnel into the vault.
As Lambert, gasping for air, finally discovers he is locked in, Virgil and Alan access Bank Station and plant explosives on a wall backing directly onto the vault.
[3] "Vault of Death" is one of several early Thunderbirds episodes that were extended from 25 to 50 minutes after Lew Grade – APF's owner, who had been highly impressed by the 25‑minute version of "Trapped in the Sky" – ordered the runtime doubled so the series would fill an hour-long TV timeslot.
Judith Shutt, one of the series' puppet operators, served as Penelope's arm and eye double for shots of the character peering through a keyhole and later snapping her fingers.