Deep space, an unknown time - on course for Gallifrey, the TARDIS passes through a strange disruption. When they materialise, the scanner shows the Time Lord homeworld... but the travellers have passed into the negative pocket universe of E-Space and are in fact on the planet Alzarius, where evolution works very differently...

As the humanoid community takes refuge in their crashed spaceship, venomous marsh spiders begin to hatch from the local fruit while bestial marshmen emerge from the waters to walk the land. It is quite apparent that the marshmen desire entry into the spaceship. Meanwhile, as a curious marshchild shadows the Doctor, a group of teen rebels decides to take over the TARDIS.

Infection from a spider bite causes Romana's personality to change. Meanwhile the Doctor, after discovering that the spaceship society is in a perpetual state of repair and upgrade, has morality issues with the ship's leading scientist who readies himself to perform gruesome experiments on the live marshchild while it's fully conscious.

Marshmen invade the Terradonian ship thanks to Romana, who seems to be in communion with them. While the ship's inhabitants flee for their lives, the Doctor discovers a crucial fact why these humanoids can never return to the planet Terradon.

Deciding to repair the TARDIS' chameleon circuit, the Doctor goes to Earth to find a real police box. But an old enemy is one step ahead of him.

The Doctor, aware that his time may soon be ending, travels to the Logopolis where a series of fateful events are set in motion.

With Nyssa from Traken unexpectedly rejoining the group, Adric aids the Monitor of Logopolis in trying to save the Doctor before time runs out. The Master, at liberty to prepare, is finally ready to reveal his true target, which unknowingly sets off the biggest catastrophe of all time.

With the universe collapsing, the Doctor forms an uneasy alliance with the Master to save it. In case all does not go well, the Watcher takes the Doctor's companions to a safe place, but his plans for them don't stop there. The one hope for all lies back on Earth.

The TARDIS lands on the jungle planet Deva Loka, where an Earth expedition has been sent to study the Kinda, a tribe of primitive people. Three team members have already vanished. Are the Kinda as primitive as they seem?

Sanders meets Panna and Karuna who leave a lasting impression on him, while the Dukkha does the same on Tegan. When Hindle's breakdown begins, the Doctor and Todd are in danger.

Aris now carries the mark of the Mara and has gained voice, so Panna shows the Doctor and Todd exactly what the past, present and future hold for Deva Loka.

As the Mara finally breaks free of its dream dimension, the Kinda must do everything they can to contain it. Hindle's need to destroy everything and everyone is unstoppable.

The TARDIS crew arrives on Earth in the 26th Century in a cave system where a military force is investigating the disappearance of a palaentological expedition.

With her crew vanishing mysteriously, Captain Briggs assumes the Doctor is the guilty party, but the traitor is closer to home… as are the Cybermen.

The Cybermen are released from their silos and are on the march, attacking Briggs's crew and ensuring that the freighter remains on course for Earth. But why?

The Doctor realises the freighter is now a huge bomb and sends it back through time where it can explode millions of years ago... but Adric is still aboard...