Sarah is marooned on a slave world where the only escape is death. Harry is caught in the middle of an interplanetary invasion, and has to combine medicine with a desperate mission. And the Doctor lands on a world so secret it does not even have a name.

Why have the TARDIS crew been scattered across the stars? What terrible accident could have wiped the Doctor's memory? And what could interest the Time Lords in this war-torn sector of space?

At the heart of a star-spanning conspiracy lies an ancient quest: people have been making weapons since the dawn of time - but perhaps someone has finally discovered the ultimate device of death.