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". 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.