A Mutant

While The Doctor is carrying out improvements to Bessie, his Edwardian Roadster, a message container, from the Time Lords, suddenly appears. The Doctor informs his companion, Jo Grant, that it will only open for one person and that the TARDIS will take them to whoever it is intended for.

The TARDIS takes The Doctor and Jo to Skybase One which is orbiting the planet Solos in the 30th Century Empire of Earth. Also on the Skybase, to attend a conference, are a party of Solonians led by the warrior Varan and Ky, leader of a rebel faction. As the visiting party are taken through decontamination, Varan's bodyguard is revealed to be in the process of transforming into a Mutt - a Mutant creature - and so is instantly killed.

The Marshal, who is in charge of Skybase One, learns from an Earth Administrator that the authorities intend to make Solos independent so ending Earth’s rule. This news horrifies The Marshal, as this will mean an end to his career, and so he plots to have the Administrator assassinated.

The Time Lords' Message

Meanwhile The Doctor and Jo, while investigating their new surroundings, are discovered by two guards and taken to The Marshal and The Administrator. Being in authority The Doctor is confused as the Time Lords’ message container does not open for either of them. The Doctor therefore requests to be able to attend the conference as the message is most likely intended for someone there, but The Marshal refuses. This though does not stop The Doctor who, on escaping from confinement, heads for the conference anyway. He arrives just as The Administrator is killed and in the confusion, Ky rushes from the room and bumps into The Doctor. Unexpectedly the container starts to open – proving that the Time Lords’ message is for him. On realising this Jo runs after Ky but ends up being taken hostage, by the rebel leader, and transported to the surface of Solos. There Ky takes her to a nearby cave system as he knows that the air on Solos is poisonous to humans during daylight.

Realising that the container is for the rebel leader, The Marshal demands that The Doctor finds away of opening it. The Doctor has no choice but to agree, as he requires the assistance of The Marshal to help him rescue Jo. The Doctor is therefore forced to work with Professor Jaeger who is working on making Solos’s atmosphere breathable to humans, regardless of the cost to indigenous life. The Marshal, meanwhile, tries to cover-up his involvement in the death of The Administrator. The Doctor then gets the opportunity to escape to the planet's surface where he starts to search for Jo.

The Marshal and The Administrator

In the caves, Jo and Ky are confronted by more of the Mutants. As Ky tries to fend them off with a flaming torch, Jo runs deeper into the cave system. She eventually finds a glowing cave and sees a radiation-suited figure before she collapses. Unknown to her The Doctor and Varan have found there way to the caves and so help Ky fend off the Mutants. The message container then opens to reveal a number of tablets marked with ancient Solonian hieroglyphs. Varan returns to his people while The Doctor and Ky try to find Jo. They are both unaware that The Marshal, who has also arrived on the planet’s surface, has ordered the sealing of all the cave exits and intends to flood the cave system with a poisonous gas.

The Doctor and Ky eventually find Jo and they also see the same suited figure, that Jo saw earlier, gesturing to them. After following him into a lead-lined room it is revealed that he is Professor Sondergaard, who has been investigating the history of Solos. He believes that the changes to the planetary environment and to the Solonians are a natural occurrence. After translating the tablets The Doctor realises that Professor Sondergaard could well be right and that the Solonians are meant to mutate every 500 years but that Professor Jaeger's experiments have accelerated the process and so must be stopped. On also discovering a strange crystal The Doctor realises that he must get back to the Skybase so that he can analyse the crystal properly. Sondergaard though decides to stay behind so that he can try and communicate with the Mutants and explain to them the changes in their metabolisms are natural and not to be feared.

Meanwhile, Varan, who has now started to mutate himself, rounds up what remains of his people with the intention of storming the Skybase. The Marshal, on returning to the Skybase, learns that an Earth Council Investigator will be arriving soon. This means that he needs Professor Jaeger to have changed the atmosphere of Solos by the time the Investigator arrives. This Professor Jaeger, with The Doctor’s earlier help, starts to carry out. But a miss-judgement means his atmospheric ion rockets hit the planet rather than exploding in the atmosphere so contaminating parts of the planet’s surface.

The Marshal

When The Doctor also returns to the Skybase he is forced to help the Marshal, who has Jo held hostage, to clean up the contaminated areas on the planet's surface. Jo though manages to escape and is able to alert the Earth Council Investigator of the problems on the Skybase and the planet. But she is caught by The Marshal who has her, along with Ky, taken to the radiation chamber where they will be killed by thaesium radiation as the Earth Council Investigator’s spaceship re-fuels.

The Doctor's plan works and Solos is decontaminated. He is then pressured into assuring the Investigator that all is well. However, when Jo and Ky escape from the radiation chamber, and burst into the meeting, the true story starts to unfold. But the Marshal continues to assert that the Mutants are diseased and must be wiped out. His argument is then confirmed when a Mutant arrives on the Skybase and disrupts the meeting. The Marshal takes the opportunity to press home his point by killing the Mutant. He then has Ky returned to the irradiated radiation chamber. In the confusion, The Doctor and Sondergaard are able to analyse the crystal in Professor Jaeger's lab and realise that it acts as a biocatalytic agent. When they manage to get the crystal to Ky he begins a process of mutation. But he transforms rapidly, first into a Mutant, and then into a glowing angel-like super-being.

After releasing Jo and the others from the radiation chamber Ky starts dispensing justice by killing the guards until he finds the Marshal whom he disintegrates. With the Investigator understanding what has been going on and with Sondergaard deciding to stay on Solos to help the Solonians fulfil their destiny The Doctor and Jo depart in the TARDIS - their mission for the Time Lords complete.