The Doctor discovers Polly and Ben aboard the TARDIS shortly before the ship arrives in 17th century Cornwall, where the local churchwarden gives the Doctor a cryptic message shortly before being murdered.

Pike is convinced that the Doctor knows the location of Avery's treasure while Polly and Ben try to convince the locals they were not responsible for Longfoot's murder.

Polly and Ben are placed in the custody of Blake, who reveals he also has his suspicions about the Squire, while the Doctor and Kewper are forced to team up to escape from Pike's ship.

The Doctor and his friends are at the mercy of Cherub, who is now seeking Avery's treasure for himself, while Pike prepares to attack the village.

In 1986 a new planet appears in the Solar System which turns out to be a long-lost twin of Earth's named Mondas. Its inhabitants are the Cybermen.

The Doctor's warnings are ignored and the Cybermen take over the base, hampering the crew's efforts to help Zeus IV.

With the Doctor out of action, it is up to Polly, Ben and Barclay to stop Cutler using the Z-Bomb and devastating half the world.

The Cybermen take over the base again, apparently to stop the humans using the Z-Bomb, but then the Doctor realises they intend to use it themselves to destroy Earth.

The TARDIS arrives on the Earth colony of Vulcan where the newly-regenerated Doctor assumes the identity of an Earth Examiner.

While the Doctor worries about the inert Daleks in the capsules, Lesterson makes plan to reactivate one of them.

The Doctor fails to convince the colonists of the danger posed by the Daleks and soon the other two are also reactivated.

With Polly being held prisoner by the rebels, the Doctor and Ben decide to investigate them, while Lesterson discovers the Daleks are reproducing.

The Doctor and Quinn attempt to escape in order to stop Bragen using the Daleks to take over the colony.

With Bragen having taken over, the Daleks trick the rebels into turning them loose and only the Doctor can stop them wiping out the entire colony.

The Doctor, Polly and Ben arrive in 1746, in the aftermath of the Battle of Culloden, and are captured by a group of Highlanders shortly before they themselves are captured by redcoats.

Polly and Kirsty capture Lieutenant Ffinch to try and enlist his help while the Doctor tricks Grey in order to escape.

With the Highlanders imprisoned aboard Trask's ship, the Doctor attempts to expose Grey's plan.

Ben manages to join up with the Doctor, Polly and Kirsty and together they work to free the Highlanders from Trask's ship before Grey sells them into slavery.

The TARDIS materialises on a deserted beach where the Doctor and his friends are captured by primitive warriors and taken to the fabled city of Atlantis.

While Ara tries to save Polly from being turned into a Fish Person and Ben and Jamie try to escape from the mines, the Doctor discovers the full extent of Zaroff's plans.

The Doctor and Ramo are sentenced to be sacrificed to the goddess Amdo and must escape to put a stop to Zaroff's plan.

With Zaroff's plan nearing completion, the Doctor and Ben realise the only way to save the world is to destroy Atlantis.

The TARDIS arrives on the Moon in the year 2070.When Jamie is injured, the other travellers are forced to seek the help of a moonbase afflicted with a space plague.

The Doctor and his friends attempt to convince the Moonbase crew that the Cybermen are responsible for the disappearances and find a cure for the plague.

The Cybermen take over the Moonbase and begin using the Gravitron to attack Earth but Polly, Ben and Jamie come up with a way to fight back.