A meeting in a London bus with jewel thief Lady Christina takes a turn for the worse for the Doctor when the bus takes a detour to a desert-like planet, where the deadly Swarm awaits.

The Doctor and Romana learn the second segment of the Key to Time is on the planet Calufrax. Yet they arrive on a planet called Zanak, which has been hollowed out and fitted with hyperspace engines, allowing its insane, half-robot Captain to materialise it around smaller planets and plunder their resources.

The Doctor is summoned before the High Council of Time Lords to stand trial for the charges of harmful interference to the course of events during his travels which have threatened the sanctity of the universe. Indignant at these accusations, the Doctor pleads his case to the Inquisitor with the hope that she will see him as a source of hope and goodwill for existence. However, his prosecuting attorney, a sinister Time Lord known simply as the Valeyard, begins a crusade against the Doctor with the motive of framing him as a villainous renegade.

The Doctor's TARDIS lands at the Snowcap space tracking station in Antarctica in December 1986. A routine space mission starts going wrong. When the base personnel's suspicions are roused, the Doctor informs them that the space capsule is being affected by the gravitational pull of another planet — a tenth planet in the Solar system.

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...

The blue crystal that the Doctor took from Metebelis III in a previous adventure is desperately sought by the Eight Legs, a race of mutated spiders, as the final element in their plan for universal domination. With help from an old mentor, the Doctor realises the only way to foil the plot is to make the ultimate sacrifice. The Doctor must risk death to return to the cave of the Great One and save the universe.

On the planet Vortis, the Doctor and his companions aid the butterfly-like Menoptra in their battle against the parasitic Animus and its mind-controlled minions, the ant-like Zarbi.

When the TARDIS malfunctions, it and its travellers land on Earth at the size of insects. They uncover a murderous plot involving a new insecticide, but how can they stop the crime in their miniaturised condition?

A strange signal from Earth draws the TARDIS to the island of Lanzarote, where Turlough rescues a young American girl, Peri, from drowning. Among her possessions is an artefact bearing an alien symbol - the same triangular mark that Turlough has branded into his arm. The mystery deepens when Kamelion falls under the control of a powerful mind, and the TARDIS travels to the volcanic world of Sarn. As Turlough is forced to face his past, the Fifth Doctor must stop his oldest enemy from harnessing the revitalising powers of Numismaton gas...

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

Actors from the show talk about the Difficulty of making The Tenth planet, And the leaving of William Hartnell

At the Tranquil Repose mortuary, the Doctor and Peri uncover a sinister plot to create a new breed of Daleks under the supervision of the mysterious Great Healer.