The Doctor and Jo arrive on the planet Solos, on a assignment by the Time Lords, where they learn that the native Solonians are mutating into ant-like mutant creatures...

The Doctor is forced to help the Marshal cover up his actions with the Investigator but Jo and the others attempt to escape to expose him.

England, the near future. A strange anti-matter creature arrives on Earth and attacks UNIT HQ. It seems to be specifically seeking the Doctor out - but why? With the Time Lords themselves unable to help, it seems the only person who can help the Doctor is himself - or selves...

The third Doctor and Jo find themselves on an artificial world inside the black hole while the second Doctor tries to find a way to control the organism.

The Time Lord Omega reveals himself to the third Doctor along with plans for vengeance against his own kind.

Omega reveals the purpose behind bringing the Doctor to his antimatter world, but the Doctor(s) discover a startling fact about Omega that he didn't know himself and eventually strike a bargain.

The Doctor attempts to test his new dematerialisation circuit by taking Jo to Metebelis Three but instead the TARDIS arrives in the hold of a 1920s cargo ship.

The Doctor and Jo attempt to find a way out of the Scope but, when Vorg attempts to entertain the Minorans, the crew of the Bernice suddenly become a lot more dangerous.

The Doctor and Jo return to the Bernice to try and aid their escape but find themselves pursued by the Drashigs.

The Doctor escapes from the Scope and joins forces with Vorg to try and save its inhabitants but Kalik is still determined to use the Drashigs to start a rebellion.

England, the 12th century. Robber baron Irongron is recruited by Linx, a Sontaran warrior whose starship has been forced down on Earth. Linx is forced to use a time-travel device to steal resources and scientists from the future to repair his ship, but in doing so involves UNIT and the Doctor...

The Doctor attempts to return the scientists to their own time, before Linx tries to use his spaceship and destroys the entire castle.

Scotland, the near future. Something is smashing oil rigs off the Scottish coast, and UNIT have been called in to investigate. The Brigadier summons the Doctor to Earth to help in the investigation, but it soon becomes clear some kind of alien presence is at work...

The Zygon traps the Doctor and Sarah Jane in a decompression chamber while Harry is taken to the aliens' underwater ship.

The Doctor and the Brigadier begin to realise who the Zygon imposters among them are while Sarah Jane discovers a secret tunnel to the underwater ship.

UNIT attempt to track down the Zygon ship while Broton prepares to have the Skarasen attack London.

The Doctor and Sarah arrive in Renaissance Italy unaware that they have brought a deadly alien energy force with them.

The Mandragora energy joins forces with the secret Brotherhood of Demnos. Count Federico plots to steal the Dukedom of San Martino from his nephew, Giuliano.

Tempted by the promises of ultimate power offered by Mandragora, Hieronymous plots to eliminate the threat posed by the Doctor and uses an unwitting Sarah as his weapon.

As the leading scholars of the era assemble in the palace for the masque, Mandragora prepares to attack, intent on sending the world into a new dark age.