The TARDIS arrives on the planet Marinus on an island of glass surrounded by a sea of acid. The travellers are forced by the elderly Arbitan to retrieve four of the five operating keys to a machine called the Conscience of Marinus, of which he is the keeper. These have been hidden in different locations around the planet to prevent them falling into the hands of the evil Yartek and his Voord warriors, who plan to seize the machine and use its originally benevolent mind-influencing power for their own sinister purposes.
The Doctor, Ian, Barbara and Susan arrive in the TARDIS on board a spaceship. Their initial concern is for the ship's human crew, who are suffering from telepathic interference from the Sensorites, but Susan communicates with the Sensorites and finds the aliens fear an attack by the humans and are just defending themselves. Travelling to the Sense Sphere (the Sensorites' planet) the Doctor seeks to cure an illness to which the Sensorites and Ian have succumbed, but finds it has been caused by deliberate poisoning. The political manoeuvring of the Sensorite City Administrator poses another threat to the TARDIS crew as he seeks to discredit and implicate them.
12-year-old David is accidentally knocked out in the forest near his home, but when he awakens eight years have passed. His family is overjoyed to have him back, but is just as perplexed as he is that he hasn't aged. When a NASA scientist discovers a UFO nearby, David gets the chance to unravel the mystery and recover the life he lost.
A spacecraft is discovered on the floor of the Pacific Ocean, presumed to be at least 300 years old and of alien origin. A crack team of scientists and experts is assembled and taken to the Habitat, a state-of-the-art underwater living environment, to investigate.
A military veteran goes on a journey into the future, where he can foresee his death and is left with questions that could save his life and those he loves.
Peggy Sue faints at a high school reunion. When she wakes up she finds herself in her own past, just before she finished school.
When a hunter sent back to the prehistoric era runs off the path he must not leave, he causes a chain reaction that alters history in disastrous ways.
The Doctor, Jamie and Zoe return to Earth and become embroiled in the schemes of Tobias Vaughn. They meet up with an old friend, Colonel (now Brigadier) Lethbridge-Stewart, and some old enemies, the Cybermen.
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 TARDIS lands in a space museum on Earth in the late 21st century, where the Second Doctor, Jamie and Zoe learn that contact has been lost between Earth and the Moon. In this era, instant travel — T-Mat — has revolutionised the Earth. Its people have lost interest in space travel. The Doctor and his companions travel to the Moon in an old-style rocket and reach the Moonbase, control centre for T-Mat, only to find a squad of Ice Warriors have commandeered the base and plan to use the T-Mat network to their advantage.
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 Second Doctor and Jamie arrive on what appears to be a space station called the "Wheel in Space" it's here that they meet Zoe Herriot a young parapsychology librarian, The Doctor hits his head concussing him leaving Jamie and Zoe to defend off the Cybermen from taking over "The Wheel in Space."
The arrival of a mysterious comet heralds impending danger from enemies both old and new. As Ace helps the Doctor defend Earth, she is confronted with a dangerous question..."Doctor Who?"
The TARDIS arrives on the planet Terra Alpha, where the Seventh Doctor and Ace discover a society in which sadness is against the law - a law enforced zealously by the brightly uniformed Happiness Patrol. The planet is ruled by Helen A with the aid of her companion, Joseph C, and her carnivorous pet Stigorax, Fifi.
London, 1963: The Doctor returns to the place where it all began — alongside his latest companion, Ace, with unfinished business. Not for the first time, unusual events are unfolding at Coal Hill School. At 76 Totter's Lane, the Doctor discovers that his oldest foes — the Daleks — are on the trail of stolen Time Lord technology that he left on Earth long ago. The Daleks are planning to perfect their own time-travel capability, in order to unleash themselves across the whole of time and space.
When two belligerent Dominators and their robotic servant Quarks land on the peaceful planet Dulkis planning to drop a radioactive seed into the planet's core to refuel their spaceship, the Second Doctor, Jamie and Zoe must attempt to inspire the pacifist Dulcians to resist.
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.
Time-traveling bounty hunters find a doomed race-car driver in the past and bring him to 2009 New York, where his mind will be replaced with that of a terminally ill billionaire.
A group of archaeological students become trapped in the past when they go there to retrieve their professor. The group must survive in 14th century France long enough to be rescued.
Hundreds of years after humans have settled on Mars, Regulator Rogul and Lord Jens Maul, lead a force of Martians to Earth in order to conquer the planet. Queen Metaphor looks to the gay heroes aboard the spaceship Surprise -- Captain Kork, Mr. Spuck, and first engineer Schrotty -- for help.