Secret of the Tower was the final story in the 1973 Doctor Who Holiday Special. It was unusual for featuring the Brigadier and the Third Doctor in the London Underground, echoing their first meeting in the television story The Web of Fear.
The Third Doctor and the Brigadier are in the midst of a massive search of the London Underground. They are trying to stop Hingrad, a master of disguise whom the Doctor describes as "the greatest thief in the universe".
UNIT soldiers are on hand to question passengers when they debark from their trains. One interviewee reacts badly to being questioned. He runs into the bowels of the Underground. The Doctor gives chase. Under Tower Hill tube station, he and the Brigadier find the man has disappeared, but the Doctor notices something wrong with the dimensions of the tunnel walls. Striking a portion of the tiled wall, he discovers a door into a secret chamber.
The Doctor is swiftly apprehended by Hingrad, who puts him in stocks in what is effectively the basement of the Tower of London. Hingrad aims a cannon at the Doctor and ignites it. The Doctor forces himself out of the cannon's line of fire, while using the cannonball to rip through the side of the stockades. Now free, he gives chase to Hingrad.
The Doctor is unable to stop Hingrad, even with the help of the Brig's men and some Beefeaters, but his chase has blown the thief's cover. No longer able to sneak around the Tower and make a quiet run at the Crown Jewels, Hingrad is left with only one option: escape via the Thames.
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