The Doctor's attempt to give Harry a trip in the TARDIS sees the two of them and Sarah Jane arriving onboard a space station in the far future, with some rather deadly security mechanisms.

The Doctor and Harry revive the Ark's medtech, Vira, to help Sarah Jane. But when station commander Noah is revived, he becomes suspicious of the newcomers.

Knowing that Noah has been infected by the Wirrn, the Doctor persuades Vira to halt the reviving of her people, knowing it will place them in danger as well.

The Doctor tries to protect the humans still in cryogenic suspension from the Wirrn, who are determined to use Earth as their new breeding ground.

The Doctor, Sarah Jane and Harry teleport to Earth to ensure the planet is safe for the survivors on board Nerva Beacon to return and reinhabit their world. A Sontaran named Styre has captured a group of humans and is conducting experiments on them to discover the human body's weaknesses, as part of the Sontaran's goal for domination of the galaxy.

The Doctor learns that Styre has been experimenting on the Galsec crewmembers to pave the way for a Sontaran invasion of Earth and decides the only way to halt his plans is to challenge him to a duel.

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.