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The Sixth Doctor (1984-1986)
Season
Twenty-One Season Twenty-One: Change, My Dear
For the first time, a new Doctor is introduced at the end of a season.
Season
Twenty-Two Season Twenty-Two: Instabilities
First appearances of Sil and the Rani.
Season 
Twenty-Three Season Twenty-Three: On Trial
First appearances of Mel, the Valeyard and Sabalom Glitz.

Season Twenty-One: Change, My Dear

The Doctor
The Sixth 
Doctor Colin Baker played the Doctor from The Caves Of Androzani in March 1984 to The Trial Of A Time Lord in December 1986. He returned for Dimensions In Time in November 1993.

The Story
The Twin 
Dilemma
The Twin Dilemma by Anthony Steven, directed by Peter Moffatt
The Doctor experiences serious regenerative instability, causing him to attack Peri and then decide to live as a hermit on the barren moon of Titan. There he stumbles upon a plot by his old friend, the Time Lord Azmael, who has kidnapped twin mathematical geniuses named Romulus and Remus. Azmael's adopted planet, Joconda, has been taken over by the sluglike Mestor and his Gastropods, forcing the Time Lord to do Mestor's bidding. But even Azmael is unaware of Mestor's true plan -- to destroy Joconda's sun, and thereby scatter Gastropod eggs throughout the galaxy.

Making History
The idea behind the Sixth Doctor was to create a character who, in sharp contrast to his generally more amiable predecessors, would initially be a source of suspicion for the audience. Consequently, the Sixth Doctor would appear to be erratic and untrustworthy -- but, in principle at least, would eventually earn the viewer's faith and respect.

Season Twenty-Two: Instabilities

The Stories
Attack Of The 
Cybermen
Attack Of The Cybermen by Paula Moore, directed by Matthew Robinson
The TARDIS is commandeered by the mercenary Lytton and a group of Cybermen, who are using another captured time machine to travel back to 1985. There, the Cybermen intend to use Halley's Comet to obliterate the Earth, thus preventing the destruction of their home planet Mondas in 1986 and perverting the course of history. Taken prisoner on Telos, the Doctor and Peri escape and ally themselves with the native Cryons. But in order to stop the Cyber plot, they may have to rely on none other than Lytton, whose motivations remain a mystery to all.
Vengeance On 
Varos
Vengeance On Varos by Philip Martin, directed by Ron Jones
When the TARDIS runs out of vital Zyton-7 ore, the Doctor makes an emergency landing on the planet Varos, which is rich in the mineral. Varos is a former penal colony whose residents now derive pleasure purely from the televised tortures which perpetually pass across their screens. The Governor of Varos is engaged in negotiations with the ruthless sluglike businessman Sil, who is trying to cheat the Varosians out of their rightful profit on Zyton-7. It is up to the Doctor and Peri to stop Sil's plans, and break the natives of Varos out of their daily cycle of video nasties.
The Mark Of 
The Rani
The Mark Of The Rani by Pip and Jane Baker, directed by Sarah Hellings
The TARDIS is drawn to Earth during the Luddite Uprisings. There, the Master is once again trying to alter the planet's history, while an evil Time Lady called the Rani is also present, extracting chemicals from the brains of local workers for her own use. As a result of the Rani's experiments, rioting amongst the workers is intensifying, threatening the work of famed engineer George Stephenson. It falls to the Doctor and Peri to foil the uneasy partnership between the two villains and restore Earth's history to its proper course.
The Two 
Doctors
The Two Doctors by Robert Holmes, directed by Peter Moffatt
The Time Lords send the Second Doctor and Jamie to Space Station Camera, to put an end to temporal experiments being conducted by Dastari, an old friend of the Doctor's. Dastari has genetically augmented a savage Androgum named Chessene, who has forged an alliance with the Sontarans. They kidnap the Doctor and take him to Seville, where they plan to isolate the Rassilon Imprimature: the genetic code which allows Time Lords to travel through the vortex. The Sixth Doctor and Peri rescue Jamie and follow the others to Seville, in a race against time with the Doctor's past and future at stake.
Timelash
Timelash by Glen McCoy, directed by Pennant Roberts
The Doctor and Peri arrive on Karfel, which is ruled by an enigmatic tyrant known as the Borad who wields the power of a space-time tunnel called the Timelash. Investigating, the Doctor learns that the Timelash connects Karfel to nineteenth-century Scotland, where a young HG Wells becomes embroiled in the Borad's schemes. Meanwhile, the Borad stokes the fires of war with Karfel's neighbours, the Bandrils. He plans to use the conflict to repopulate Karfel with beings such as himself: a hideously mutated cross between a human and a reptilian Morlox. And Peri will be but the first...
Revelation Of 
The Daleks
Revelation Of The Daleks by Eric Saward, directed by Graeme Harper
The Doctor and Peri go to Necros to attend the funeral of an old friend of the Doctor's. There they discover that Davros is posing as the Great Healer of Tranquil Repose, a famed institution where the terminally ill can be placed in suspended animation until a cure for their ailment is found. Davros is experimenting on the comatose bodies to produce a new race of Daleks loyal to himself. To defeat his old foe, the Doctor may have no choice but to ally himself with the original Daleks on Skaro.

Making History
Season Twenty-Two saw something old and something new for Doctor Who as the show returned to its old Saturday evening timeslot but was expanded to forty-five minute episodes rather than the traditional twenty-five minute length. Despite this, things did not fare well for the programme; ratings were down slightly on recent years and there were renewed calls that Doctor Who was becoming too violent. Combined with the general lack of funds at the BBC in the mid-Eighties and a disregard for Doctor Who by certain administrators, this prompted BBC management to put the programme on hiatus until September 1986, resulting in the cancellation of the stories originally planned for Season Twenty-Three. Furthermore, it was made clear that, when it did return, Doctor Who would have to justify its existence, or be removed from the schedules again -- permanently.

Season Twenty-Three: On Trial

The Companions

Melanie Bush, or Mel as she preferred to be called, was a computer programmer from Earth gifted with an eidetic memory.

Bonnie Langford played Mel from The Trial Of A Time Lord in November 1986 to Dragonfire in December 1987. She returned for Dimensions In Time in November 1993.

Mel 
Bush

The Production Team
In the midst of all the turmoil of Season Twenty-Three, the disagreements which had been brewing for years between John Nathan-Turner and Eric Saward finally came to a head following the death of Robert Holmes, to whom Saward had become quite close. Fed up, Saward quit the programme, forcing Nathan-Turner to assume the dual roles of producer and script editor for the concluding episodes of the season.

The Stories
The Trial Of 
A Time Lord (Segment One)
The Trial Of A Time Lord (Segment One) by Robert Holmes, directed by Nicholas Mallett
The TARDIS is summoned by the Time Lords to a space station. There, the Doctor, suffering from amnesia and unable to remember what happened to Peri, is put on trial again for interfering with other cultures. The Doctor elects to act as his own defense attorney; his prosecutor is a grim figure who calls himself the Valeyard. Using the Matrix, the Valeyard shows the court an adventure of the Doctor and Peri on Ravolox in the far future. The surface of Ravolox was supposed to have been destroyed by a solar flare, but instead the time travellers find it perfectly habitable and home to the barbaric Tribe of the Free, led by the warlike Queen Katryca. The companions soon discover that Ravolox is actually Earth, having somehow been moved light years across space, and that a second race of people lives underground, governed by the robot Drathro. Also on the planet are two conmen, Glitz and Dibber, who have come to steal Drathro's secrets. But Drathro is dying, and his passing will set into motion a chain of events which will tear Ravolox apart.
The Trial Of 
A Time Lord (Segment Two)
The Trial Of A Time Lord (Segment Two) by Philip Martin, directed by Ron Jones
At the Doctor's trial by the Time Lords for interference, the Valeyard presents the Doctor and Peri's most recent adventure. The dying words of a Thordon warlord send the pair to Thoros Alpha, home of the Mentors -- including their old foe, Sil. The Mentor leader, Kiv, has had his intelligence enhanced by the human geneticist Crozier, but now his brain is outgrowing his skull. Crozier sets his eyes on Peri as the new host for Kiv's brain. But when the Doctor appears to turn evil under the effects of one of Crozier's devices, it is left to the berserk warlord King Yrcanos to save his companion.
The Trial 
Of A Time Lord (Segment Three)
The Trial Of A Time Lord (Segment Three) by Pip and Jane Baker, directed by Chris Clough
It is the Doctor's turn to argue his case against the Time Lord's accusations of meddling at his trial. He presents an adventure from his future when he is travelling with an Earth computer programmer named Mel. Summoned for help by his old friend Captain Travers, it is up to the Doctor to solve a series of murders happening on board Travers' ship. Amidst a web of genetic manipulation and political maneuvering, the Doctor discovers that the botanical experiments of a scientific team on the ship has resulted in the creation of a new race of monsters, the Vervoids, who will stop at nothing to destroy all non-plant life.
The Trial 
Of A Time Lord (Segment Four)
The Trial Of A Time Lord (Segment Four) by Robert Holmes and Pip and Jane Baker, directed by Chris Clough
As the Doctor's trial by the Time Lords for interfering in galactic affairs nears its conclusion, the Master appears in the courtroom, speaking from within the Matrix. He reveals that the entire trial is the result of a conspiracy by the corrupt High Council. Drathro's creators had stolen Matrix secrets and fled to Earth; in revenge, the High Council ravaged the Earth, rechristened it Ravolox, and changed its position in space to cover up their deeds. More importantly, the Valeyard is not who he seems -- he is actually the Doctor, or more precisely the distillation of the Doctor's evil side between his twelfth and final regeneration. The Doctor and Mel, brought to the trial by the Master, pursue the Valeyard into the Matrix and discover he is plotting to destroy the High Council. The Master has also summoned Glitz, however, and allies himself with the crook in his own bid for power.
At the conclusion of the trial, it is revealed that Peri has elected to wed King Yrcanos. Mel travels on with the Doctor.

Making History
When Doctor Who finally returned from its hiatus, restored to standard twenty-five minute episodes, John Nathan-Turner was given explicit orders to tidy the show up or see it cancelled. Consequently, the Trial concept was formulated to parallel Doctor Who's own plight. Ratings after the hiatus were not good, however, amounting to even less than what Season Twenty-Two had attracted. Blaming Colin Baker's portrayal of the Doctor, BBC management fired the actor at the season's conclusion, but decided to renew Doctor Who for one more year. The programme was not out of hot water yet by any means...