Ace is dead. Or at least she will be — soon... In a secret room deep inside the TARDIS the Doctor has been examining the body of Ace's future self. He now knows how she was killed, where she was killed and when she was killed. What he doesn't know is why...

To find the truth the Doctor makes a dangerous decision and takes the unsuspecting Ace to the very time and place of her death, hoping to cheat time and find her killer before he can strike — but time has other ideas. With Ace missing and the clock ticking the Doctor turns to old friends for help and finds that there is unfinished business for him to deal with.

What is the secret experiment being conducted by the British Rocket Group? Why are giant ants appearing in the suburbs of 1950s London? Who is the mysterious figure that is watching the Doctor's every move?

As events spiral out of control the Doctor realises that someone is manipulating time with careless disregard for the consequences to Ace — or the rest of the universe...