A Device of Death by Christopher Bulis

In A Device of Death, the crew of the TARDIS have been scattered to different planets with Sarah marooned on a slave world where the only escape is death. Harry is in the middle of an interplanetary invasion and the Doctor is on a planet that is so secret it doesn't even have a name.

To complicate matters further the Doctor has lost his memory and 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.