Two burglars are fatally injured trying to rob a flat owned by Beth Halloran (Nikki Amuka-Bird) and her husband. Torchwood is called in because it all seems strange. The police think Beth killed them, but she has no memory of it. When she is put in a cell at the Torchwood Hub, the Weevil in the next cell is terrified of her. Jack orders a mind-probe. They discover alien tech buried in her right forearm, which allows it to transform into the kind of blade that killed the burglars. Jack believes she is an alien sleeper agent.
Other such agents are awakened in Cardiff, suddenly killing people. Three are implanted with bombs that destroy a comm centre and a military pipeline and kill members of the Cardiff Council. Torchwood leaves the Hub and allows Beth to escape, so they can follow her. She goes to the hospital and stabs her husband. Jack and Gwen apprehend her, and she agrees to help them.
It seems the agents are targeting a military base where nuclear weapons are stored. Jack stops an agent there, who stabs him. He warns that there are other such agents on Earth before his implant explodes. Beth is distraught and uses her blade to threaten Gwen so they will kill her.
The explosion of a building to be demolished was used in the filming. The first time the hospital was used for a scene was for Spearhead from Space, with Jon Pertwee as the Doctor, in 1970. Beth’s morgue drawer is 007. Nikki Amuka-Bird, a Nigerian actress, appeared in the Doctor Who Christmas Special “Twice Upon a Time.” She played the glass woman.