The TARDIS arrives on the space liner Empress which has become locked together with a private ship, the Hecate, after colliding with it on emerging from hyperspace.
The Doctor and Romana meet the scientist Tryst, who has with him a Continuous Event Transmuter (CET) machine containing crystals on which are stored supposed recordings of planets that he and his team have visited.
Someone on board the liner is smuggling the dangerously addictive drug vraxoin, and to complicate matters the interface between the two ships allows some monstrous Mandrels from the mud-swamps of Eden to escape from the CET machine - which does not merely take recordings but actually displaces whole planetary areas into its crystals.
The smugglers are revealed to be Tryst and the Hecate's pilot, Dymond. Vraxoin is in fact the material into which the Mandrels decompose when they are killed. The Doctor thwarts this plan, separates the two ships and returns the Mandrels to Eden.
The Armageddon Factor
The final segment of the Key is traced to the planet Atrios, engaged in a long war with the neighbouring Zeos. The Marshal of Atrios intends a final strike to destroy the Zeons, but the Doctor and Romana discover that Zeos is deserted and the war is being co-ordinated by a computer called Mentalis, built by one of the Doctor's old Time Lord Academy friends, Drax.
The computer is under the control of the Shadow, an agent of the Black Guardian. He and his servants, the Mutes, are inhabiting an unseen third planet positioned between Atrios and Zeos.
The Doctor creates a temporary substitute for the final segment from a substance called chronodyne and uses the Key to place a time loop around the ship from which the Marshal is about to launch his strike against Zeos. It transpires that Atrios's Princess Astra is the real sixth segment. The Shadow converts her into the segment, but the Doctor snatches it and escapes to the TARDIS, where he finally completes the Key.
The White Guardian appears on the scanner screen and congratulates the Doctor. He asks that the Key be given over to him, but the Doctor decides that it is too powerful for any one being to control and orders it to re-disperse.
Enraged, the Guardian reverts to his true colour - Black - and vows that the Doctor shall die for his defiance. In order to shake him off, the Doctor fits a randomiser to the TARDIS's controls. There is now no telling where or when his travels will take him.
The Invisible Enemy
The TARDIS is infiltrated by the Swarm - a space-borne intelligence that wishes to spread itself across the universe - and the Doctor is infected by its nucleus. The ship then materialises on Titan, one of the moons of Saturn, where the human occupants of a refuelling station have also been taken over.
The Doctor eventually collapses as a result of his infection, but first manages to relay to Leela the coordinates of a local hospital asteroid. At the Bi-Al Foundation, based on the asteroid, Professor Marius clones the two time travellers, miniaturises the clones using the relative dimensional stabiliser from the TARDIS and then injects them into the Doctor's body in the hope that they can find and destroy the nucleus.
The plan backfires as the nucleus escapes from the Doctor in place of the clones and is enlarged to human size. The creature arranges for itself to be taken back to Titan, where breeding tanks have been prepared for it.
The Doctor, now cured of its influence, enlists the help of K9, Professor Marius's dog-shaped robot computer, and sets a booby-trap that results in the breeding tanks being blown up, killing the nucleus. Marius gives K9 to the Doctor as a parting gift.
The TARDIS arrives on a Minyan space craft, the R1C, commanded by a man named Jackson. Jackson and his crew are on a long quest to recover the Minyan race banks from a ship called the P7E which left their planet centuries ago. The Doctor helps to free the R1C after it becomes buried in a meteorite storm, but it then crashes into another newly-formed planet.
Inside the planet is a system of caves at the heart of which is the P7E. The P7E's computer, the Oracle, was programmed to protect the race banks but subsequently went insane and - with the aid of its robotic servants, the Seers - imposed its rule upon the Minyan survivors and their descendants. It allows Jackson to take what appear to be the race banks, but they are actually imitations containing fission grenades.
The Doctor realises the deception and obtains the genuine race banks. He then tricks the Oracle's guards into taking the grenades back to their leader. The resulting explosion destroys the planet and the P7E and boosts the R1C off on a voyage to Minyos II, carrying with it the Minyan survivors.
The Hand of Fear
The TARDIS arrives on contemporary Earth, where Sarah comes into contact with what appears to be a fossilised human hand. This is in fact the last surviving fragment of a Kastrian called Eldrad, who was blown up in space as a punishment for attempting to wipe out his own people.
Eldrad's essence lives on in a blue-stoned ring, which the possessed Sarah removes from the hand and places on her own. She then takes the hand to the nearby Nunton nuclear research and development complex where it soaks up the radiation from the reactor core and regenerates into a complete being.
Eldrad - who, having patterned his new body on Sarah's, appears to all intents and purposes female - persuades the Doctor to take him back to Kastria where he might reclaim his heritage. There he discovers only a dead planet: the Kastrians are extinct and their race banks were destroyed by their last King, Rokon, in case Eldrad should ever return.
Furious, Eldrad, whose body has now been reconfigured into its proper form, tries to get the Doctor to return him to Earth so that he might rule there instead. The two time travellers use the Doctor's scarf to trip him into a deep crevasse.
Back in the TARDIS, the Doctor receives a summons to return to Gallifrey and, as he cannot take Sarah there, endeavours to drop her off at her home.
The Sontaran Experiment
The Doctor, Sarah and Harry arrive on a desolate and apparently deserted Earth to discover that a group of shipwrecked astronauts from a human colony, GalSec, have been lured there by a fake distress call. One of their number, Roth, tells Sarah of an alien conducting gruesome experiments on him and his crewmates. The alien turns out to be a Sontaran, Field-Major Styre, who is compiling a report on human physical and mental capabilities as a prelude to an invasion of Earth.
The Doctor challenges Styre to unarmed combat. The Sontaran agrees but is quickly weakened in Earth's unfamiliar gravity. Harry meanwhile enters Styre's ship and uses the Doctor's sonic screwdriver to remove the vital terullian diode bypass transformer, so that when the alien returns there to revitalise himself he is drained of all his energy and destroyed.
The Doctor sends a message to the Sontaran fleet, warning them that without Styre's report they cannot invade.
The Three Doctors
A gel-like plasma creature arrives on Earth and hunts down the Doctor, who calls on the Time Lords for help. The Time Lords themselves are in crisis as their energy is being drawn off into a black hole. They send the Doctor's earlier selves to join him.
The first Doctor, caught in a time eddy and able only to advise, deduces that the creature is a time bridge. The third Doctor and Jo then give themselves up to it and are transported to a world of antimatter beyond the black hole.
On Earth, the second Doctor is forced to take refuge in the TARDIS along with the Brigadier and Sergeant Benton. On the advice of the first Doctor he switches off the ship's force field, and the whole UNIT building is transported through the black hole.
Behind these events is Omega, a figure from Time Lord history whose solar engineering provided the power for time travel. He has been trapped in the black hole ever since and now wants the Doctor to swap places with him, but it transpires that the corrosive properties of his domain have already destroyed his physical form, leaving only his will.
He threatens to destroy the universe but is tricked into touching the second Doctor's recorder - the only thing not converted to antimatter when the TARDIS passed through the black hole - and is consumed in the resulting supernova. Everyone else is returned home.
The Time Lords send the Doctor and Jo on a mission to deliver a sealed message pod to an unknown party aboard a Skybase orbiting the planet Solos in the 30th Century. Solos is due to gain independence from Earth's empire, but its Marshal is determined to prevent this. He arranges the murder of the Earth Administrator and, with his chief scientist Jaeger, plans to transform Solos's atmosphere into one more suited to humans.
Ky, a young Solonian leader, is falsely accused of the murder, and flees to the planet, taking Jo with him. The Doctor follows and joins them in an old thaesium mine. Ky turns out to be the intended recipient of the message pod, which opens automatically for him. Inside are stone tablets carved with ancient inscriptions.
The Doctor's party then meet Sondergaard, a human scientist leading a hermit-like existence in the mine while searching for a cure for the mutating disease that afflicts the Solonians. The Doctor and Sondergaard decipher the inscriptions, deducing that the mutations are part of a natural life-cycle in which the thaesium radiation plays a vital role.
The Doctor retrieves a crystal from a cave where the radiation is concentrated and returns to the Skybase to analyze it. He is recaptured by the Marshal and, with his friends held hostage, is forced to perfect the machine with which Jaeger plans to transform Solos. Sondergaard meanwhile gives Ky the crystal, which turns him first into a mutant, and then into an ethereal super-being - the ultimate stage of the Solonians' life-cycle. Jaeger is killed when the Doctor sabotages his machine, and the Marshal is vaporised by Ky.
The Claws of Axos
An approaching alien spaceship is detected on monitoring equipment at UNIT HQ, where the Brigadier is entertaining two visitors - Chinn, a civil servant making a security inspection, and Bill Filer, an American agent sent to discuss the threat of the Master. The ship lands in England and the UNIT team, joined by Hardiman and Winser from the nearby Nuton power station, meet its occupants: beautiful golden-skinned humanoids called Axons.
The Axons claim that their ship, Axos, is damaged and that they need time in which to repair it. In return, they offer Axonite, a substance that can cause animals to grow to enormous sizes and thus end food shortages. The Doctor is suspicious, and rightly so: Axos, Axonite and the Axons - whose true appearance is hideous - are all part of a single parasitic entity brought to Earth by the Master to feed on the planet's energy.
The Doctor manages to materialise his TARDIS, with the Master on board, at the centre of Axos. He offers to link the two ships together to make one giant time machine, on condition that Axos in return helps him to take revenge on the Time Lords for exiling him to Earth. This is merely a trick, however, and Axos is locked in a time loop from which it can never escape.
The Doctor returns to Earth in the TARDIS, where he reluctantly admits to the Brigadier that the Master may also have escaped.