Finding the fourth segment of the Key to Time was simple enough, but holding onto it may be another matter. The Doctor and Romana find themselves embroiled in the political games of the planet Tara, where doubles, android or otherwise, complicate the coronation of Prince Reynart.
Doctor Who: The Invasion of Time (1978)
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.
Doctor Who: The Pirate Planet (1978)
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.
Doctor Who: The Stones of Blood (1978)
Searching for the third segment to the Key to Time brings the Doctor and Romana to present-day Earth, where the travellers have to contend with stone circles, Druidic rituals and a not-so-mythical goddess known as the Cailleach.
Doctor Who: Castrovalva (1982)
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...
Doctor Who: Four to Doomsday (1982)
The Doctor and his companions arrive on a spaceship headed for Earth, populated by natives of Earth from various different eras and commanded by the leaders of the Urbankan race. What are the Urbankans' intentions when they reach Earth?
Doctor Who: Kinda (1982)
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.
Doctor Who: The Visitation (1982)
In 17th century England, the Doctor and itinerant thespian Richard Mace uncover a plot by a crew of criminal Terileptils to wipe out humanity.
Doctor Who: Black Orchid (1982)
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.
Doctor Who: Time-Flight (1982)
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.
Doctor Who: Arc of Infinity (1983)
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!
Doctor Who: Snakedance (1983)
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.
Doctor Who: Mawdryn Undead (1983)
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?
Doctor Who: Terminus (1983)
The TARDIS attaches itself to a space liner after Turlough, still under the Black Guardian's influence, damages its controls. The Doctor and Nyssa meet two space pirates, Kari and Olvir, who have come on board the liner in search of plunder, while Tegan and Turlough get lost in the infrastructure. The liner docks with what appears to be a hulk floating in space. This is Terminus, which claims to offer a cure for Lazar's disease. It is crewed by armoured slave workers, the Vanir. The cure is administered by a huge, dog-like creature known as the Garm. Nyssa, who has contracted the disease from sufferers transported aboard the liner, discovers that the cure - involving exposure to radiation - does actually work.
Doctor Who: Enlightenment (1983)
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.
Doctor Who: The King's Demons (1983)
The Doctor and his companions arrive at a medieval joust and are surprised to be greeted warmly by King John, who calls them his demons. But when a young nobleman returns, having just left King John in London, the Doctor realises that this king must be an impostor! Then the Master makes an appearance and the Doctor's worst fears are confirmed...
Doctor Who: Warriors of the Deep (1984)
The Fifth Doctor, Tegan and Turlough arrive on Sea Base 4, a nuclear warhead station under the sea that has some very nasty neighbours.
Doctor Who: The Awakening (1984)
The Doctor, Tegan and Turlough arrive in Little Hodcombe, where the townspeople's re-enactments of English Civil War battles are causing a dormant entity, the Malus, to re-awaken.
Doctor Who: Frontios (1984)
"Frontios buries its own dead", or so the saying goes. The Doctor, Turlough and Tegan are forced into landing on the remote planet of Frontios, a human colony where deaths go unaccounted for. What lies beneath the surface, dragging its victims down?
Time Tracers (1997)
When the discovery of a five-thousand year old artifact of a humanoid reptile leads an investigative reporter to the Kronos Project, he uncovers an experimental device created to send humans through time, controlled by a ruthless multi-billionaire. As the Kronos experiment progresses through the Civil War era to the Jurassic age, high-tech sabotage and deceit ensue, resulting in devastating events that lead to a ripple in time, a paradox, that if not contained could change the entire course of evolution and destroy the universe as we know it.
Doctor Who: The Power of Kroll (1979)
The Doctor and Romana arrive on the marsh moon of Delta Magna in search of the next segment of the Key to Time. They are caught in the conflict between the native Swampies and the crew of a chemical refinery. The presence of a gun runner complicates matters; to make things worse, the Swampies intend to awaken Kroll, the giant god that lives beneath the swamps.