When scientists in the Antarctic uncover a mysterious seed pod, the Doctor is called in to investigate. He soon realises it is extraterrestrial and extremely dangerous. At the same time, however, ruthless millionaire plant-lover Harrison Chase has learned of the find and decides he must have the pod for his collection of rare and beautiful flora. Meanwhile the pod itself harbours intelligent life with sinister plans of its own...

Arriving on the barren world of Androzani Minor, the Doctor and Peri find themselves embroiled in a long running, literal underground war. At the heart of the conflict is a substance called Spectrox - both valuable and deadly! The Doctor & Peri wind up being poisoned by the material, which is killing them slowly and painfully unless they can find a cure. As the conflict heats up and the situation gets more desperate, the Doctor realises time is running out - both for Peri and himself...

In the distant future, the Doctor, Sarah and Harry find a space station preserving the future of mankind. But the insect Wirrn have got there first...

Routine tests on a traffic accident victim lead to shocking discoveries when the man's blood is found to be unidentifiable and x-rays reveal a disc embedded in his brain. His fabulous tale of being an escaped prisoner from an alien spaceship takes a turn for the sinister when the hospital staff realise that they're under a state of siege...

Arriving on Space Station Nerva in its distant past, the Doctor, Sarah and Harry find its crew threatened by a mysterious plague. Discovering that things are not as they seem, they uncover a Cyberman plot to destroy Voga, planet of gold.

The Doctor is summoned by the mysterious and powerful White Guardian, and sent on a quest to find the six segments of the Key to Time, which, once assembled, will restore balance to the Universe. Joining the Doctor and K9 is the smart and sassy Romana, a Time Lord fresh from the Academy. Landing on the wintry planet of Ribos to locate the first segment, the TARDIS crew quickly find themselves embroiled in a little local trouble with a pair of conmen and an unstable warlord...

The TARDIS materialises in Earth's future on a space beacon just before it is attacked by pirates. The travellers find themselves trapped in a sealed section of the beacon. It is blown apart and flown to where the pirates will plunder it of the precious mineral argonite. They witness a conflict between the pirates and the Interstellar Space Corps, led by General Hermack and Major Warne. The ISC are convinced that the pirates' mastermind is an innocent yet eccentric space mining pioneer named Milo Clancey, while their true leader is a man named Caven. Caven has a secret base on the planet Ta. He is assisted by Madeleine Issigri, daughter of Clancey's ex-partner Dom, who - unknown to her - is now his captive. When Madeleine discovers Caven's full treachery, she helps to bring him to justice. The time travellers are given a lift back to the TARDIS by Clancey in his rickety old ship, the LIZ 79.

The New Doctor arrives on Earth in the middle of a freak meteor shower. Faceless killer shop window dummies are coming to life and killing anything that comes into their path while Facsimiles are replacing top Military, Political and Civil leaders. The Nestene has come to invade the Earth. The convalescent Doctor teams up with Brigadier Lethbridge-Stewart and Dr. Elizabeth Shaw of UNIT to thwart the Nestine's plans even as the invasion begins.

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 Master, an old adversary of the Doctor, has arrived on Earth to destroy humanity with the aid of the Nestene Consciousness and their army of Autons and killer plastic daffodils - unless the Doctor, joined by new assistant Jo Grant, can stop them.