The Doctor is contacted by the White Guardian who tasks him with finding the six segments of the Key to Time. Joined by a young Time Lady named Romana, he begins his quest on the planet Ribos.

The Doctor and Romana investigate Garron's activities but this results in the Graff believing they are working together.

The Graff takes the Doctor, Romana, and Garron prisoner and then sets about trying to track down Unstoffe and the jethryk.

The Doctor returns to the city to keep an eye on the Graff, leaving Romana, K9 and Garron to search for Unstoffe and the first segment.However, Garron double crosses them and steals the tracer.

In seeking the second segment of the Key to Time, the TARDIS heads for an icy world but lands on a populated planet that shouldn't be there.

The Doctor and Kimus head to the Bridge to try and rescue Romana, leaving K9 and Mula to search for Pralix.

The Mentiads take the Doctor, Romana and Kimus to a reunion with K9 and Mula, where they set about trying to defeat the Captain's plans.

The Doctor reveals the truth about the real power on Zanak and must try and stop the Captain and Xanxia before Earth becomes their next victim.

The Doctor and Romana's search for the third piece of the Key to Time leads them to a stone circle in Cornwall where they find signs of strange activity and meet the inquisitive Professor Rumford who tells them of a nearby sect of druids.

Rumford arrives just in time to save the Doctor from being sacrificed, allowing him and K9 to rescue Romana from the cliff face but he is confused when she claims that she was pushed by someone who looked just like him.

Assisted by Prof. Rumford and K-9, the Doctor builds a projector by which to enter hyperspace and rescue the kidnapped Romana, all of them all the while trying to avoid the bloodsucking Ogri stalking them.

Aboard a hyperspace prison ship, two justice machines called the Megara, known for swift verdicts and harsh punishments, arrest the Doctor for breaking an infraction, with trial and execution to be carried out immediately.

The Tardis materializes on the Earth-like planet of Tara. The Doctor goes fishing while Romana finds the fourth segment of the Key to Time but the powerful Count Grendel mistakes her for an android and brings her to his castle.

After Count Grendel kidnaps the real Prince Reynart, the Doctor suggests his android lookalike be crowned king in his absence. Back at the Count's castle, Romana is introduced to Princess Strella who looks exactly like her.

While the Doctor works on the android Prince Reynart's speech circuits, Count Grendel realizes that Romana is a friend of the Doctor and plans to lure him into a trap using an android with her face.

In a last attempt to take the throne, Count Grendel plans to have Romana wed to Reynart with the intent to kill him afterwards and take his place as her husband. When the Doctor arrives to foil his plan, the Count challenges him to a duel.

The Doctor and Romana land on the moon Delta III, home to the green-skinned Swampies and a small group of human colonists. Soon after arriving, Romana is taken by the Swampies who wish to sacrifice her to the mysterious squid-like Kroll.

The Doctor rescues Romana from the Swampies but is horrified to hear that Kroll has awoken.

The Doctor, Romana and Rohm-Dutt are sentenced to death by the Swampies while the refinery crew try to work out a way to destroy Kroll.

The Doctor races to stop Thawn from destroying the Swampie settlement and then works to destroy Kroll once and for all.

The Doctor and company drop in on the planet Atrios, whose inhabitants are in an interplanetary war with their twin planet Zeos. Although Astra, Princess of Atrios, wishes for peace, the Marshal, her commander of the military, does not share her views. So, while the TARDIS is in orbit around the planet, the Marshal targets the Doctor for destruction.

Unable to escape with the Tardis missing, the Doctor, Romana and Merak return to the Marshal who has an unusual change of heart and asks the Doctor for help but it soon becomes clear that someone or something else is controlling him.

The Shadow kidnaps the Doctor and tries to secure the Key to Time, while the Marshal prepares for an all-out assault against Zeos. Meanwhile, the Doctor's companions try to locate the missing TARDIS.

The Doctor sends Shapp and Merak back to Atrios to try and halt the Marshal's attack, while he and Romana attempt to disable Mentalis.

With both K9 and Princess Astra under his control, the Shadow lures the Doctor and Romana to his planet where the Doctor reunites with his old Time Lord friend Drax.

Romana looks on as Princess Astra turns herself into the sixth segment of the Key to Time, giving the Shadow ultimate power. The Doctor and Drax arrive just in time but for the Doctor one final trial remains.