The Doctor is chosen for a quest by The White Guardian to go in search for the six segments to the Key to Time, that have been scattered and sent throughout the time/space continuum. With the Key to Time, The White Guardian will restore balance to the universe. Joined by a young time lady named Romana, The Doctor, Romana and K-9 Mark 2 begin their quest and arrive at their first destination, The medieval Russian like world Ribos, where a conman named Garron bids to sell Ribos to a exiled tyrant known as Graff Vynda K, where Unstoffe, Garron's assistant uncovers Garron's scheme and is unaware he is in possession of the first segment of the Key to Time.

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.

Continuing their quest in search of the Keys to Time. The Doctor, Romana and K-9 travel to the cold planet "Calufrax" in search of the second segment to the Key to Time. But the TARDIS accidentally lands on the planet Zanak, which is ruled by the robotic space pirate The Captain. Only to discover Zanak can travel across the galaxy and nobody on Zanak, isn't who they appear to be.

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.

On Earth to collect the third segment to the Key to Time, the Doctor and Romana encounter modern day druids in Cornwall making blood sacrifices to Cailleach, the goddess of war and magic, at a megalithic circle known as The Nine Travelers. The Tracer says the third segment is there, then says it isn't, which is very odd.

The Doctor and Romana's lives have already been imperiled. Now megalithic creatures from planet Ogros begin to attack under the directives of a mysterious master. Investigating the history of the area, Romana notices that for centuries only women have owned the land and nearby manor and wonders if there's any significance to this.

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.

When Romana goes fishing for the fourth segment to the Key to Time, the Doctor just goes fishing. While on the planet of Tara, both are taken in (somewhat against their will) by two factions, both trying to achieve the throne of Tara. While Romana is mistaken for an android by Count Grendel, the Doctor and the Prince are drugged after the Doctor agrees to help repair his android.

Discovering that they've all been drugged, the Doctor tries to escape and passes out just as he opens the door, envisioning the evil Count Grendel as he loses consciousness. When he awakes, he is, once again, at the tip of a blade, and his life threatened by those who believe he is behind all of it, while the Count has kidnapped Prince Reynart. Meanwhile, back at Count Grendel's castle, Romana finds herself an exact double for Princess Strella, whom the Count plans to marry once he ascends to the throne so he can claim her land for his own (once he kills her, too).

The Doctor kills the android princess, Count Grendel finds his plans foiled and can not find an opening to kill the new king (especially since the King's head bodyguard now suspects him of treachery against the King). So he offers Romana up in exchange to the Doctor for his own safety. But the Count has built another android programmed to kill the Doctor once it hears the Doctor's voice. Meanwhile, the real Romana escapes Grendel's castle and tries to ride for help.

The Count has offered the throne to the Doctor, just before killing the android King and riding off with the, once again, captured Romana, all under a flag of truce. As the Doctor and K-9 plan a rescue effort for Romana and the King, Count Grendel plans to marry Romana to the King, kill the King, marry Romana, then kill her, as well, inheriting all of the King's land and powers...

The TARDIS arrives at it's next destination, the marsh planet "Delta Magna". Where the Doctor and Romana goes in search of the fifth segment to the Key to Time. Where they discover the native tribal Swampies worships Kroll, a gigantic squid as a god, only to find Kroll has accidentally swallowed the fifth segment and Romana is chosen by the Swampies for sacrifice and The Doctor sets out to save a local refinery from being attacked by 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.

In an effort to escape the Marshal, the Doctor finds the TARDIS buried under rubble, and figures that he may have stumbled across the missing princess, who has now been abducted by someone else. But he does find a new ally in her love, who believes that the Doctor did not have anything to do with her disappearance.

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.

The Shadow, now with control of K-9, awaits the arrival of the Doctor and company in the cave where he took control of the princess. Meanwhile, Princess Astra joins Romana and the Doctor in the TARDIS, where he had locked the Marshal in a continual time loop just before attacking Astrios. Returning to the cave where Princess Astra was held captive, K-9 pursues the Doctor, while Astra captures Romana.

The Doctor, having met another Time Lord named Drax in the caves, teams up with him to try and defeat the Shadow, but Drax shrinks him instead of his Mute captor then turns the shrinking ray on himself, allowing the Shadow to claim the Key to Time. Meanwhile, everyone finds out that Princess Astra, herself, is the sixth and final segment of the Key to Time. And once it is assembled, nothing stands between the Shadow and absolute power of the Universe...except the Doctor!