On Gallifrey, a Time Lord traitor is at work, stealing the bio-data code of another Time Lord and killing a technician who stumbles across the crime. The traitor provides the bio-data to a creature known as the Renegade, which is composed of anti-matter and uses the bio-data to invade the TARDIS and then the Doctor's metabolism. His companion, Nyssa, helps him recover. The Renegade is shielded in this attempt by the Arc of Infinity, a curious curve between the dimensions containing quad radiation, which can shield anti-matter. The Doctor decides to head to Gallifrey to track down the supplier of his bio-data, conscious that unless the creature trying to cross universes is stopped, its incursion could cause a chain reaction fatal to our universe.