Four to Doomsday — Terrance Dicks

When the TARDIS materializes on an alien spacecraft, the commander of the ship -- the reptilian Monarch -- invites the Doctor, Adric, Nyssa, and Tegan to continue their journey to Earth in his company. Monarch's hospitality even extends to a generous offer to liberate the time travelers from the shortcomings of their bodies and replicate them as androids -- so much more practical. Although Adric finds this proposal extremely attractive, the Doctor has good reason to be suspicious of Monarch's motives. Unabridged.