A man staggers towards the River Thames near Kew Railway Bridge, wearing ragged clothes and a strange metal helmet. He screams, tearing the straps off the helmet, walks into the river, and drowns himself. The TARDIS materialises, the Doctor surmising from the surroundings that they have landed in London. Ian and Barbara are delighted they have returned home, but it is curiously silent and deserted. The Doctor is concerned about the decay they see around them, fearing that it is nowhere near Ian and Barbara's time. Susan tries to look over a wall, but slips and twists her ankle. At the same time the bridge under which the TARDIS has materialised collapses, burying the TARDIS and blocking the travellers' access to it. The Doctor and Ian look in a nearby warehouse for tools to help unblock the TARDIS door while Barbara looks after Susan.
Ian sees Battersea Power Station with its chimneys damaged, while the Doctor finds a calendar marked 2164. Barbara, who went to the river to soak a handkerchief for Susan's ankle, returns to find her gone and a dirty-faced man there instead. The man tells Barbara that they have to get out of there before they get killed, and that Susan has been taken by someone called Tyler. He urges Barbara to follow him, as the sound of automatic fire is heard in the distance. Meanwhile, the Doctor and Ian stumble across a corpse wearing the same strange metal helmet seen earlier. They find an electronic receiver attached to the helmet and a whip on the body, and discover that the man has been stabbed to death. They decide to return to the others but spy a flying saucer hovering over the city.
Barbara follows the man across the ruins of London. Tyler carries Susan down a flight of steps into an empty Underground station as Barbara catches up. Susan and Barbara demand that they find their friends, but Tyler says that there is no time. Tyler activates a secret panel and David Campbell emerges from it, telling Tyler that he had a struggle with a Roboman and that they will have to tell Dortmun to change the storehouse. He reveals that he saw the Doctor and Ian in the warehouse but thought they were enemies. Dortmun, a man in a wheelchair, arrives, and Tyler tells him that a saucer has landed at the heliport. Dortmun says that this time they will be ready for "them." He is glad to see Susan and Barbara as extra sets of hands. David goes to find the Doctor and Ian; Dortmun asks him not to be long, as they need to go through the attack plans. Tyler takes Susan and Barbara below while Dortmun stays on watch, armed with a knife.
Ian and the Doctor arrive back at the TARDIS to find the women gone. David spots the Doctor and Ian from a window, but also sees several Robomen converging on them. The Doctor and Ian find themselves surrounded, but the Robomen only tell them to stop. The two try to run away, but turn to see the distinctive form of a Dalek rising from the river.