Can the Doctor, now in his fourth incarnation, recover from his post regenerative trauma to save the Earth from Think Tank and their plot for world domination ?

The Doctor and Sarah Jane become convinced Think Tank are behind the electronic component thefts but are unable to stop them carrying out another raid.

Sarah Jane and Kettlewell infiltrate an SRS meeting to try and find out what Miss Winters is up to but there is a surprise in store.

UNIT attempt to get into the bunker to stop Miss Winters setting off the world's nuclear missiles but first they have to get past the K1.

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 planet Skaro, the distant past. The Time Lords divert the Doctor, Sarah Jane and Harry to this war-torn world, and give the Doctor a mission: the evil of the Daleks is about to be born here, and the Doctor is to try to ensure they evolve into less lethally aggressive beings - or, failing that, to avert their creation altogether. But has even the Doctor the right to commit genocide?

The Doctor and Harry are taken to the Kaled bunker where Davros is preparing to demonstrate the Daleks, while Sarah Jane is captured by the Thals and taken to their city as a slave worker.

The Doctor fails to prevent the Thals from destroying the Kaled city but they don't have long to enjoy their victory before the Daleks attack.

The Doctor is forced to give Davros full details of future Dalek victories but opposition to Davros' plans is growing among the Kaleds and soon there is open rebellion.

Gharman tries to convince the Kaleds to vote against the Dalek project but Davros has a trick up his sleeve, while the Doctor works to destroy the tape recording of Dalek victory and the Thals plan an intervention of their own.

The orbit of Jupiter, the 25th century. Arriving back on the Nerva Beacon much earlier in history, the Doctor and his friends discover the satellite beset by space plague while its crew struggles to keep space traffic away from Voga, the newest moon of Jupiter. But what is the connection between Voga, the plague, and some old enemies of the Doctor?

The Doctor transmits Sarah Jane and Harry down to Voga to cure Sarah Jane of the Cybermat's poison but Kellman's sabotage means he is unable to bring them back.

The Cybermen force the Doctor, Stevenson and Lester to carry bombs to the centre of Voga while Sarah Jane, Harry and Tyrum decide to contact Vorus.

The Doctor, Harry and Lester try to destroy the Cybermen's relay device, while the Vogans prepare to destroy Nerva even though Sarah Jane is still on board.