The Time Lords dispatch the Doctor to the past of the planet Skaro on his deadliest adventure yet — to prevent the creation of the Daleks.

To escape from the volcanic eruption on Dulkis, the Second Doctor uses an emergency unit. It moves the TARDIS out of normal time and space. The travellers find themselves in an endless void where they are menaced by white robots. Having regained the safety of the TARDIS, they believe they have escaped — until the ship explodes. They find themselves in a land of fiction, where they are hunted by life-size clockwork soldiers and encounter characters like Rapunzel, the Karkus, and Swift's Lemuel Gulliver.

The TARDIS arrives on the unnamed planet of the Gonds, who are ruled and taught in a form of self-perpetuating slavery by the alien Krotons — crystalline beings whose ship, the Dynatrope, crash-landed there thousands of years earlier after being damaged in a space battle. The Krotons are in suspended animation, in a crystalline slurry form, awaiting a time when they can be reconstituted by absorption of mental energy. Periodically, the two most brilliant Gond students are received into the Dynatrope, nominally to become "companions of the Krotons", but in truth to have their mental energy drained, after which they are killed.

The Doctor, Jamie and Zoe arrive on an unnamed planet. At first believing themselves in the midst of World War I, they realise it to be one of many War Zones overseen by the War Lords, who have kidnapped large numbers of human soldiers to form the greatest army the universe has ever seen. At the helm of this plot is the War Chief, another renegade Time Lord like the Doctor. The creeping realisation sets in that the Doctor cannot solve this problem alone, and that his days of wandering may be at an end...

On the jungle planet Spiridon, the Doctor, Jo and a Thal group find a slumbering army of Daleks ready to awaken.

In the 26th century, the Doctor and Jo uncover a plot by the Master to provoke an interplanetary war between Earth and Draconia.

Summoned back to Gallifrey, the Doctor is framed for the assassination of the Time Lord President. To prove his innocence, he must stand for election himself, uncover the traitor in the Time Lord High Council, fight a hooded killer in a nightmare cyberspace world of his enemy's devising and battle against an old foe now out to destroy the Time Lords: The Master.

Picking up a distress call from the edge of the known universe, the Doctor and Sarah Jane find themselves on Zeta Minor where a geological team has run afoul of some strange goings on...

Death stalks the fogbound streets of Victorian London: young women are going missing, horribly mutilated bodies are found floating in the Thames and criminal gangs terrorize the innocent. At the heart of this tangled web sits the mysterious Li H'sen Chang, sorcerer and hypnotist, and his grotesque sidekick Mister Sin. The Doctor dons deerstalker hat and cape to seek out the sinister force lurking in the shadows of the metropolis, for the Talons of Weng-Chiang are reaching out to shred the human race.

The final segment of the Key to Time is at the heart of a devastating war between neighbouring planets Atrios and Zeos. The Fourth Doctor discovers that a sinister entity is manipulating events and the cost of obtaining the final segment may be more personal than he imagined.