Investigating a series of mysterious deaths in Cardiff, Jack (John Barrowman) recruits UNIT medical specialist Martha Jones (Freema Agyeman), who was once, like Jack, a Doctor Who companion. The problem seems to be toxic shock, but the victims’ medical records have been deleted, and Owen (Burn Gorman) and Martha think they have been murdered. What’s more, they all had an incurable disease like diabetes and HIV but were cured of them before they died. They had all been participants in a clinical trial at the Pharm, run by Professor Copley (Alan Dale). The last to die, in the team’s presence, has flies coming out of her mouth.

Jack and Owen contact Professor Copley, who denies any problem, but Jack detects alien life-forms and the Pharm has a security system not even Torchwood can hack. Martha offers to be a test subject and claims to have hepatitis. She is given something called Reset that releases mayfly-like alien parasites that cure the diseases, but the parasites kill the host when they hatch. Copley can tell that Martha has travelled in Time and realizes she is working with Torchwood.

The Pharm’s assassin is caught about to eliminate the last living test subject. Taken to the Hub, he agrees to help Torchwood rescue Martha, but it appears he too is infested with mayflies. Owen tries to fix this but ends up killing him. They put the dead assassin behind the wheel of his car and use it to fool the security guards and gain entrance to the Pharm. They find many tormented aliens being used to produce test substances, including mayflies. They find Martha nearly dead, but Owen is able to save her. Jack has Tosh (Naoko Mori) shut down the Pharm, euthanizing the suffering aliens and crashing all the computers. They are stopped at gunpoint by Professor Copley. Owen takes a bullet for Martha Jones. Jack kills Copley, but Owen dies as well.

This is the first of three Torchwood episodes featuring Martha Jones. In the early stages of the script, it was Ianto who was shot, not Owen. Does anyone think he won’t be back?