There is a creature loose on the war-torn planet of Tersimmon. It has just destroyed an entire race, and others are dying too. Only one person will defend this killer: because Jamie McCrimmon knows the Doctor isn't himself.
Life in the colony of Sora is idyllic, thanks to a device implanted in all citizens which helps memory recall. As a visitor, Jamie receives one too. But soon the Doctor has concerns about his friend which lead to the office of Dr Hunter.