Two prisoners meet in a prison cell. Zara is searching for the segments of the Key to Time; she was only born yesterday but already she’s killed hundreds of people. Ace is more ambitious: she was going to kill everyone on the planet. What have they got against the people of Erratoon? They go peaceably about their simple assignments, beneath their artificial sky. They share their meals and leisure time and they never ask questions. Are they even real? Ace and Zara will only survive if they can trust each other. Or perhaps if they sell each other out... If not their awful punishment is to become just like everyone else.

A new adventure in time and space for the Fifth Doctor and his new companion, Amy as they search for the Key to Time. On a planet where Time stands still, the Doctor meets a woman who is just a few minutes old. She is a Tracer, sent into our Universe by her makers to locate the six segments of the Key to Time. This being without a name wants the Doctor to be her assistant, but she doesn’t tell him the whole truth. Not at first. Their first port of call is Mars, where a society that one day will become Ice Warriors lives in peace and civility. But the Doctor’s arrival will change all that. The universe is dying, a choice must be made, and the Judgement of Isskar will be declared. The price must be paid - even if it takes centuries…

“You will be always looking in the wrong place. I have searched through all of Time and I cannot find it.” The search for the Key to Time has stalled: the next segment does not appear to exist anywhere in the Universe. Forced into a temporary alliance with one of his greatest enemies, the Doctor suggests a course of action that is a validation of chaos itself. Thrown at random across Space and Time, the Doctor and Amy arrive in 9th Century Sudan, where the greedy Lord Cassim is hoarding gold from the Legate of the Caliph. But why does Cassim look so familiar? What is the mysterious Djinni that lives out in the desert? And why does it need so much treasure?

The ageless leader of a dying race believes that salvation lies within The Chaos Pool, a place that even the Guardians of Time have been unable to locate. Meanwhile Commander Hectocot and his Teuthoidian followers move in for the kill - again and again and again... Two different races from opposite ends of Time - so how can they co-exist? In their search for the final segment of the Key to Time, the Doctor and Amy become caught in the crossfire. As the end of everything approaches, old friends and enemies reveal themselves and the final battle between the forces of Chaos and Order ignites…