Jon Pertwee
The Mutants

The Time-Lords send the Doctor and Jo on a mission to deliver a sealed message pod to an unknown party on board the Skybase orbiting Solos in the 30th century. Solos hopes to gain independence from Earth, but the Marshal is determined to stop this from happening.

The Marshal has the Earth administrator killed, and a Solonnian named Ky is blamed. Ky flees to Solos with Jo as a hostage. The Doctor follows them to Solos and meets them inside a thaesium mine. Ky is the intended recipient of the Time Lord pod, and it opens for him, revealing stone tablets with carved inscriptions.

They meet up with an Earth scientist who was investigating the mutating disease that affects the Solonnians. He has found that the thaesium radiation is in fact part of the natural life cycle. The Doctor returns with a crystal back to the Skybase, where he is captured by the Marshal, and forced to work on a machine that will destroy Solos's atmosphere.

The Marshal is killed when Ky first transforms into a mutant, and then into an ethereal being, which is the final stage in the life cycle.