Parts: 6
Writers: Bob Baker and Dave Martin
Airdates: 8 April 1972 - 13 May 1972
Production Code: NNN
Creature Feature: Mutants
Story: On a misty planet with an inhospitable atmosphere, an old man races for his life through the tall brush. He is pursued by three black-uniformed men with atmospheric masks and blasters...and as soon as they catch up to him, they gun him down. As one spitefully kicks the dead body, another reports back to their base that they successfully tracked one of the native mutants...and found it dead on arrival.
Meanwhile, in his lab, the Doctor is taken by surprise when a Time Lord message box materializes in front of him. He explains to Jo that he cannot open it, because the message is not meant for him; instead, he is the intended porter. He and Jo board the TARDIS, which is directed to the thirtieth century, as Earth's empire is beginning to collapse. They materialize on board a skybase orbiting the planet Solos, whose inhabitants are fighting for independence from Earth.
The Doctor soon finds that his message box is intended for Ky, one of the Solonians. But his attempts to reach Ky are hampered by the Marshal of the skybase, who is maniacally determined to wipe out the natives of Solos and make the planet habitable for an Earth colony over which he plans to rule. He excuses his actions by claiming that the Solonians have been infected by a plague which is causing them to mutate into ant-like creatures...but the Doctor isn't so sure. What actually is causing the Solonians to mutate? Can the Marshal be stopped from executing his genocidal plans? And what is inside the message box which only Ky can open? For the Doctor to discover the answers, he must play along with the Marshal's plans, both to buy himself some time...and to keep Jo alive.