When someone tries to assassinate diplomat Sir Reginald Styles, UNIT investigate the possibility his attacker came from the future.

Jo uses the guerrillas' time machine to travel to the 22nd century where the Controller tricks her into telling the Daleks where she's come from.

The Doctor forces the guerrillas to take him to the 22nd century where he is soon captured by the Controller.

The guerrillas infiltrate the Daleks' base to rescue the Doctor and Jo and ask them to return to their own time and alter history before the 20th century is plunged into a Third World War.

The Doctor and Sarah answer a distress call and find themselves on Zeta Minor, the last planet of the known universe, where a Morestran expedition has gone missing.

The Doctor realises that Zeta Minor lies on the boundary between the universes of matter and anti-matter and something on the planet won't let the Morestrans leave.

The Doctor makes a deal with the Anti-Matter Beast to return all the anti-matter to the planet but Sorenson is determined to continue his research.

The Doctor tries to convince Sorenson to sacrifice himself for the rest of the crew but Salamar's attempt to deal with the situation causes their problems to multiply.

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.