“When the TARDIS happens to materialise 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-travellers 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…”
Four to Doomsday was a novelisation based on the 1982 television serial Four to Doomsday. [tardis.wikia.com]