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In both the original run and since the 2005 revival, the long-running British science fiction television programme Doctor Who has featured a number of story arcs. A story arc is an extended story which takes place over a large number of episodes or serials, characterised by a gradual unfolding of plot points and introduction of narrative devices which come together in the arc's climax and dénouement. While character arcs exist over the course of many stories, they do not necessarily take the shape of an expanded story arc.
References to these arcs have since been made in both Doctor Who and its spin-offs such as Torchwood. Early seasons would feature story arcs that made up extended serials such as The Trial of a Time Lord but also loose umbrella titles such as The Key to Time.
The classic series sporadically featured loose story arcs at several times in the series, often spanning half of a season or a full season. Several story arcs were later released on DVD as boxsets; for example, The Key to Time
The Key to Time is the umbrella title that links all six serials (26 total episodes) of Season 16 of Doctor Who. The arc was originally conceived of by producer Graham Williams, who had proposed it as part of his application for the producer's job in 1976. The arc was released in both European and North American markets as a DVD box set on 24 September 2007.
The name refers to the powerful artifact that maintains the equilibrium of the universe. The Key's power is vast and considered too great for any single being to possess, so it normally exists split into six segments that are hidden across the universe, their true nature disguised from most others. The White Guardian, fearing that the balance of the universe is coming undone, requests the Doctor, Romana and K-9 to locate the six parts for him so that he can temporarily stop time in the universe and restore the proper order. The White Guardian gives the Doctor a special wand that will guide the TARDIS to each piece and, on touching the piece, reveal its true nature; the Guardian also warns the Doctor of the Black Guardian, his counterpart that seeks the Key for his own purposes.
The first segment is disguised as a chunk of mineral rock (called Jethryk) on the planet Ribos. The second is the planet Calufrax, shrunk to miniature size by the space-hopping pirate planet Zanak. The third is the Great Seal of Diplos, which has been stolen by a criminal of that planet. The fourth is part of a statue on the planet Tara. The fifth has been consumed by the squid Kroll, causing it to turn into a gigantic monster. The final segment is actually a living person, Princess Astra. In the final episode, the Black Guardian, disguised as the White Guardian, attempts to take the Key from the Doctor. However, the Doctor sees through the figure's charade and orders the segments of The Key to Time to once again become scattered across all of time and space, bar the sixth, which he reinstates as Princess Astra. Afterward, the Doctor decides to install a device called a randomiser into the TARDIS navigation system for a period of time to make his following voyages unpredictable to evade the Black Guardian.
Elements of the Key to Time return in later works. The Black Guardian returns in the 1983 three serial arc, The Black Guardian Trilogy, part of Season 20, involving the Fifth Doctor, Nyssa, and Tegan, and introducing new companion Vislor Turlough, an agent of the Black Guardian sent to kill the Doctor. Key 2 Time is a series of CD audio plays released by Big Finish Productions. Over the course of three stories, the Fifth Doctor must hunt down the redistributed segments of the Key, which are decaying the dimensions around them. The Doctor soon discovers the decay is a direct result of his actions in the first search for the Key, his attempt to assemble the Key using a makeshift sixth segment disrupting the balance between the segments and causing them to decay. The story ends with the Key being reassembled and the Doctor subsequently destroying the Key in the Chaos Pool where it was first made, undoing the damage it has caused and preventing anything from ever using the Key again. The three stories are The Judgement of Isskar, The Destroyer of Delights and The Chaos Pool.
Within Season 18 the stories Full Circle, State of Decay and Warriors' Gate form the sub-arc known as The E-Space Trilogy. The trilogy saw the TARDIS accidentally pass through a Charged Vacuum Emboitment (CVE) from its normal universe, N-Space, into a smaller one called E-Space. The trilogy served to introduce Adric as a companion in the first story Full Circle and marked the departure of companions Romana and K-9 Mark II in the final story Warriors' Gate.
The Trial of a Time Lord is the title of the fourteen-part serial that encompasses the entirety of Season 23. In the serial, the Sixth Doctor stands accused of transgressing the First Law of Time, with adventures from his past, present, and future examined and used as evidence of guilt or innocence. As the case unfolds, it becomes clear that the Doctor is the victim of an elaborate conspiracy, with his memories of recent events blurred due to the circumstances of him being removed from time, the evidence against him being either doctored or missing crucial information, and his prosecutor, the Valeyard, having an undetermined final objective. The story concludes with the Doctor defeating the Valeyard- identified as the manifestation of his own evil nature- and being cleared of all charges against him.
Fenric was an ancient evil since the dawn of time who used "wolves", descendants of a Norsemen tainted with Fenric's genetic instructions, in order to set traps for The Doctor. The Doctor and companion Mel Bush encounter Ace on Iceworld on the planet Svartos, who explains to Mel that she is a human who failed a Chemistry exam yet was swept up to Svartos by a "time storm" when experimenting with explosives in her bedroom. In Silver Nemesis, the Doctor moves pieces on a chess board in the study of one of that serial's antagonists, Lady Peinforte. The plot lines converge in The Curse of Fenric, where Fenric and the Doctor reveal that Ace's arrival on Svartos and the chess set in the study were "pawns" used by Fenric to interfere in the Doctor's life.