On the planet Alzarius in the alternate universe of E-Space, the Doctor, Romana and K9 find survivors of a crashed starliner, strange creatures lurking in the marshes and a genetic riddle which stretches back centuries.

The Doctor, Romana, Adric and K9 encounter the enslaved time-sensitive Tharils, who lead them to a white void occupied only by the ruins of an old building and a spaceship - a dimension that might be the key to escaping E-Space.

The Doctor and Adric learn from the wizened Keeper of Traken that a great evil has come to his planet in the form of a Melkur - a calcified statue. The Keeper of Traken is nearing the end of his reign and seeks the Doctor's help in preventing the evil from taking control of the bioelectronic source that is the keystone of the Traken Union's civilisation.

After an encounter with the Master, an airline stewardess named Tegan Jovanka becomes an unwitting stowaway aboard the TARDIS as it travels to the planet Logopolis. There, the Doctor discovers that the Master's interference with the Logopolitans' advanced mathematics has unleashed a wave of entropy which threatens to consume the entire universe.

When his girlfriend Giorgia leaves him for his boss, journalist Giulio suddenly finds himself without a woman and with no job. Determined to win back his ex, he attends Valeria's classes, who teaches single men the art of seduction, convinced that relationships between people are motivated by simple biological stimuli.

A three-man rocket crew are nearly done with their mission to Titan Base until a course change puts the rocket in the path of a strange cloud in space. By the time they arrive, they have come under the control of a sentient virus which threatens the galaxy. When the TARDIS picks up an emergency message, it flies into the cloud, infecting the Doctor. To save himself and others, he must undertake a dangerous journey.

The present day: just as the Fourth Doctor and Leela arrive in Fetchborough, England, Professor Fendelman prepares to experiment on a fossilized skull which science says should not exist. The skull is actually an artefact of the Fendahl, a god-like being who feeds on the life force of others. It has begun to awaken and kill. Worse yet, others seek to exploit the Fendahl's dreadful power.

On the edge of the universe, the Doctor, Leela and K9 encounter a Minyan ship on an epic quest to find their race banks... but their people have encountered the Time Lords before...

The Doctor returns to Gallifrey, claims his rights and is inaugurated President. It soon turns out that he has led a group of aliens called Vardans to the planet to eradicate them completely. After they are destroyed, the Sontarans take their opportunity and follow them in their invasion until they are destroyed by the Doctor.

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 needs somewhere peaceful to recover from his traumatic regeneration. But the sanctuary of Castrovalva is not all it seems, as the Master will stop at nothing to gain his revenge over the Doctor...

The TARDIS lands on the idyllic jungle world of Deva Loka, which is being surveyed for possible Earth colonisation. Deva Loka is already home to a race of apparent savages, however: a mysterious people with strange powers which have mentally unbalanced the members of the expedition. To make matters worse, an ancient enemy of the natives known as the Mara is intent upon revenge, and latches onto Tegan's mind as its bridgehead to victory.

In 17th century England, the Doctor and itinerant thespian Richard Mace uncover a plot by a crew of criminal Terileptils to wipe out humanity.

The TARDIS arrives in 1925 England where the Doctor ends up playing in a local cricket match due to a case of mistaken identity. The travellers accept an invitation to a masked fancy dress ball, but events take on a more sinister tone as murders are perpetrated at the country home of their host, Lord Charles Cranleigh.

While investigating a vanishing Concorde at Heathrow Airport, the Doctor and his companions are thrown millions of years back in time, when a mysterious alien called Kalid is trying to control the ancient powers of the Xeraphin.

Omega, an ancient Time Lord made of pure anti-matter, once defeated by the Doctor, is plotting to cross over into this dimension by bonding with the Doctor. Meanwhile, the disappearance of a man in Amsterdam piques the curiosity of his cousin, Tegan, who previously left the Doctor at Heathrow Airport and now finds herself at Omega's mercy. Fearing total destruction from the collision of matter and antimatter, the Time Lords recall the Doctor to Gallifrey to undertake the only viable solution: executing him!

When a son attempts to reconcile with his father, a series of fateful events allow him to experience the father's life in the past.

Tegan falls once more under the influence of the Mara and directs the TARDIS to the planet Manussa. There, the Federator's son Lon and his mother Tanha are preparing for a ceremony to celebrate the banishment of the Mara five hundred years earlier. The Mara takes control of Lon and uses him and Tegan to obtain from Ambril, the Director of Historical Research, the 'Great Crystal' - the large blue stone that originally brought it into being by focusing energy from the minds of the planet's one-time inhabitants. The Mara now plans to use the crystal during the ceremony to bring about its return to corporeal existence.

A warp ellipse draws the TARDIS off course. The Fifth Doctor's companions are separated from him not in space, but in time, and he has to deal with a treacherous schoolboy named Turlough. But why does the Doctor's old friend, the Brigadier, not remember him at all?

An Edwardian yacht in deep space races around the planets. There is a double agent in the TARDIS crew. The White Guardian warns the Fifth Doctor of great danger. Turlough must finally choose sides and at the end of the race lies the prize of Enlightenment.