Doctor Who: Four to Doomsday — Terrance Dicks

When the TARDIS happens to materalise on an alien space craft the commander of the ship, the reptilian Monarch, invites the Doctor and his companions 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 is proposal extremely attractive, the Doctor has good reason to be suspicious of Monarch's motives...