At a dance hall in 1930 New York, Tallulah flirts with her boyfriend Laszlo, who leaves her a flower before every performance. After she goes on stage, Laszlo sees something go by the door and down the hall to a storage room. He goes in and is attacked by a pig-headed human.
The Doctor and Martha land in New York a little later on Staten Island beneath the Statue of Liberty. They notice a newspaper mentioning disappearances in Hooverville, a shanty town, and go to investigate. They meet Soloman, an African-American who is trying to bring home to the Hooverville inhabits, who are poor and destitute after the Wall Street crash.
The Empire State Building is under construction and Mr. Diagoras is the man in charge. When he tells his foreman to rush the construction of the building, particularly the mooring mast, the man balks and Diagoras calls up one of his “masters.” The master is a Dalek, accompanied by pig-slaves. Diagoras clearly knows who he’s dealing with, and the Dalek hauls the protesting foreman away. The Dalek tells Diagoras to get more men for it and he agrees.
Soloman explains to the Doctor and Martha that Hooverville residences have been disappearing but the police aren’t interested. Diagoras arrives and offers work for men to clear the sewers, and the Doctor agrees along with Martha, Soloman, and a homeless boy, Frank. In the sewers they don’t find a collapse but they do locate a green brain-shaped blob of protoplasm.
Diagoras meets with his other men and tells them to complete the mast that night, despite the danger of working in the dark and cold. He gives them special metal plates, clearly made of Dalek side armor, to attach to the mast. After they leave, the Dalek returns and tells Diagoras it’s time for his reward.
The Doctor and the others hear pig-like grunting noises in the sewer and find a pig-slave cowering in a corner. He doesn’t attack them but others come after them and they flee for the surface. They go up a ladder and escape, except for Frank who is captured and taken away. Tallulah is there and points a gun at them, demanding answers. The Doctor quickly convinces her they’re friends and she puts down the (prop) gun. She explains how Laszlo disappeared even though someone still leaves off a flower every night. The Doctor starts gathering components from the theater equipment to make a DNA scanner, while Soloman goes back to Hooverville to rally the men to defend themselves.
Diagoras is taken to the four remaining Daleks, the Cult of Skaro. Dalek Sec, their leader, announces that their “final experiment” is almost ready and Diagoras will be rewarded by being used as the test subject. The other three Daleks object but Sec opens his casing and absorbs Diagoras then pulls him into the casing and shuts it.
Tallulah begins her song while Martha looks on, and she spots a pig-slave watching from the shadows. Martha follows him into the sewers and Tallulah goes after them. The Doctor realizes that the bio-sample is from Skaro and goes after Martha. Martha is captured by the pig-slaves and taken away. The Doctor finds Tallulah and confronts the pig-slave and realizes he’s a transformed Laszlo. Laszlo explains he managed to escape before he went through mental processing and has been secretly watching his girlfriend. The Doctor arrives and tells Tallulah to go back while he and Laszlo go after Martha.
One of the Cult Daleks scans Martha and the other captive humans, including Frank. Determining she is of superior intelligence, it orders Martha to the main lab for experimentation. The Doctor and Frank find her and blend in with the other captives. In the laboratory, the Doctor tells Martha to ask what is going on. Dalek Sec proclaims that a new day is dawning for the Daleks as they can emerge from their casing into the sunlight. His casing opens up to reveal that Diagoras now has a Dalek mutant merged atop his head.