The Tenth Doctor and Martha arrive in New York City in November 1930 during the Great Depression, landing the TARDIS at Liberty Island. They find a newspaper article about recent disappearances and travel to Hooverville, a tent city in Central Park. There they meet Solomon, the leader, who explains more about the disappearances. A wealthy businessman named Mr. Diagoras appears in Hooverville to recruit workers for sewer construction. The Doctor, Martha, Solomon, and a young man named Frank sign up. They are taken to the sewer and instructed to clear a collapsed tunnel. As they explore the tunnels the Doctor finds a mass of alien organic matter that he takes with him to analyse later. The group soon runs into a group of Pig Slaves and are forced to flee but Frank is captured by the creatures.
The Doctor, Martha, and Solomon use a nearby ladder to escape and find themselves in a theatre where they meet Tallulah, a showgirl whose boyfriend Laszlo is one of the people who have disappeared. The Doctor uses equipment in the theatre to create a matter analyser while Martha helps to console Tallulah. When Tallulah goes on stage for her show, Martha spots a Pig Slave across the stage. Martha gives chase into the sewers, where she is captured by more Pig Slaves. The Doctor and Tallulah follow and find the Pig Slave Martha was chasing, who they determine to be Laszlo. They also encounter a Dalek, confirming the Doctor’s analysis that shows the organic matter being from the planet Skaro. The three follow the Dalek and learn from Laszlo that the Daleks are causing the disappearances, rounding up humans to either make into Pig Slaves or use for an unspecified experiment.
Tallulah returns to the theatre while the Doctor and Laszlo sneak into the incomplete Empire State Building and locate Martha and Frank. The group encounters the Cult of Skaro, and the Doctor stays in the background while Martha asks the Daleks what they are planning to do. The Daleks reveal that they are attempting to merge the Dalek and human races. Dalek Sec conducts the first experiment on himself, fusing himself to the body of Mr Diagoras and becoming the first Dalek-human hybrid.