An antimatter creature has crossed into normal space via a phenomenon known as the Arc of Infinity but needs to bond physically with a Time Lord in order to remain stable. A traitor on Gallifrey has chosen the Doctor as the victim. The High Council, headed by President Borusa, decides that the Doctor's life must be terminated in order to avoid this danger. Tegan meanwhile arrives in Amsterdam to visit her cousin, Colin Frazer, only to learn that he has disappeared. She enters a crypt in search of him and is captured by a hideous creature, the Ergon.