When the Doctor catches up with the thief, he finds a group of youngsters thieving to survive. The thief falls through the ice and isn't seen again. The Doctor discovers that other people have fallen through the ice and never seen again.
The Doctor and Bill don deep sea suits and go underwater to find out what has been happening to the people who fall through the ice. The Doctor discovers a giant creature underwater which is eating the people and is chained up, being held against its will.
After returning to the surface, the Doctor returns to the group of thieves where they reveal the truth. They had been luring people to the spot after being paid by Lord Sutcliffe. The Doctor pays Lord Sutcliffe a visit where the Lord reveals he's been feeding people to the creature and using the waste from the creature as a fuel that has been making him rich. Sutcliffe plans to break the ice so that everyone on the ice falls through and is eaten by the creature. The Doctor and Bill break free of the chains and rush to the river to get everyone off before the bomb goes off.
Sutcliffe is lost under water, the only casualty so the Doctor and Bill return to the TARDIS and then to the present. Nardole is angry that he's been left behind whilst the Doctor went off on a journey. Nardole visits the vault and talks to who is behind it.