Turlough has returned to his homeworld after a successful counter-revolution has overthrown the insane dictator Rehctaht. As an Imperial Clansman who openly protested Rehctaht's rule and willingly went into exile rather than work for her, Turlough has been welcomed back to Trion and his career is now full of promise. However, he has bewildered his supporters -- particularly his former friend Juras Maateh -- by ignoring his political responsibilities and instead touring archaeological sites, apparently investigating the history of the Laima, an intelligent species which lived on Trion before the humanoid species evolved. He visits the vast chasm known as the Giant's Drop, the oddly-named Mobile Castle, and the Slotsruin, an odd artefact protected by a force barrier. The Slots, the other intelligent life form on Trion, tend to keep to themselves, and in his time on the Slotsisland, Turlough only sees one Slot there. When Juras confronts Turlough, he claims to have found evidence linking the New Regime's allies, the mysterious Gardsormr, to the dictatorship of Rehctaht -- and that the Gardsormr, as well as visiting Trion, have also visited two other planets. And one of them is Earth.
Turlough departs from Trion to conduct secret experiments in deep space, only to find, too late, that Juras has stowed away on board. He no longer trusts her completely, as she chose to remain on Trion and work in Rehctaht's research division while he stood up for his principles and was exiled to Earth. She claims that she chose to do so in order to divert Rehctaht's research into more productive, less destructive realms. Rehctaht was obsessed with understanding gravity, believing that it was the key to the Unified Field Theory and thus understanding of time travel. Juras has already guessed what Turlough intends to do; using the information he gathered while travelling with the Doctor, he intends to build a time machine of his own. With Juras' help, he succeeds, travelling to the planet Regal for supplies to find only ruins and a pleasant, eccentric young man who claims that the planet was mined out thousands of years ago.
Satisfied, Turlough takes his ship back to Earth to collect his notes on the Gardsormr's movements, and Juras is amused by his revelation that the people of Earth think it's impossible to travel faster than the speed of light. She is puzzled, however, when she tries to take some readings based on her understanding of gravity waves, only to find her calculations all coming out wrong. The eccentric young man from Regal then unexpectedly arrives, revealing himself to be a Time Lord called the Magician, and claiming that he's here to observe Turlough's progress. With Juras' help, Turlough has already constructed a time machine using energetic muons in sequence -- ARTEMIS -- and together, they have the potential to make the final scientific breakthrough and prevent the entropic decay of the Universe. Turlough, uncertain whether the Magician is here for his own amusement or as a representative of the Time Lords with orders to stop him from developing time travel, decides to take the Magician with him while following the Gardsormr.
Using his notes, Turlough manages to predict the location of a Gardsormr ship, but he is spotted following them and the Gardsormr capture his ship, sending robot mini-drones to capture the occupants. Turlough and his fellow travellers lose consciousness and awaken on a forest world, the third planet visited by the Gardsormr, with a colony of Trions who claim to have been abducted and stranded here for no apparent reason. Turlough is disgusted to learn that, rather than work together to solve their problems, they have engaged in a war with another settlement of Trions from a rival continent. Turlough and Juras are forced to start working on a plan to use radioactive materials from an ancient, alien castle to build nuclear bombs, but the Magician grants Turlough a vision which reveals the alien castle to be the Mobile Castle of Trion -- which logically means that it must somehow be capable of time travel. With this knowledge, Turlough and Juras are able to divert their work and fool their captors into believing that they are building a stockpile of nuclear weapons, when in fact they are building an ARTEMIS drive to convert the Mobile Castle into a ship capable of interstellar flight. Summoning the colonists to the Castle, ostensibly for a test of their nuclear weapons, they launch the Castle into space and take the colonists with them, heading back to Trion.
Turlough uses the ARTEMIS drive to prove to the suspicious colonists that their future on "New Trion" held only war and death, but is surprised when, following a nuclear exchange on New Trion, the Gardsormr arrive and begin building domes on the planet's surface. He pilots the Mobile Castle back to Trion, where he is once again greeted as a returning hero -- until the refugees claim that he told them the Gardsormr were responsible for their predicament. Turlough is now an embarrassment to those who allied themselves with the Gardsormr, and he is summoned before the Committee of Public Safety to explain himself and to hand over the blueprints for his ARTEMIS drive. Before he can decide what to do, the orbit of the Castle begins to decay, and the Trion authorities are unable to shoot it down for fear of causing nuclear fallout to rain down over the planet. Turlough, Juras and the Magician travel to the Castle to try to prevent its collision with Trion, but Juras suddenly attacks the Magician and Turlough. They are able to overpower her and realize that post-hypnotic commands were implanted in her brain during her time in Rehctaht's laboratories; but the delay has cost them. The Castle's drives have been sabotaged, and it is too late for them to do anything about it. They are forced to take the Castle's ARTEMIS drive back to their shuttle, and flee as the Castle falls from orbit and collides with Trion, devastating the planet. Shortly afterwards, the Gardsormr arrive and begin building domes on the shattered remains of Turlough's homeworld...
Turlough pilots his time-ship back to Earth, seeking answers, only to find that it too has been devastated by a nuclear war triggered by an asteroid collision, and that Gardsormr domes have been planted there as well. Upon landing, Turlough is startled to see a Slot -- the same Slot he met during his tour of Trion -- emerging from one of the domes. The Slot claims to have been protected by the force field of the Slotsruin and says he is here to protect both the past and future of his race, but he departs without explaining himself further. Turlough enters one of the domes to find deformed telepathic humanoids slaughtering giant, foul-smelling slugs and cutting up their remains. At first he assumes the humanoids to be the mutated survivors of the war, but then he, Juras and the Magician encounter a band of survivors led by David Swallow, who claim that the mutants -- the "Pharix" -- arrived on the planet along with the domes, and that the remains of the Land Snails are being used as fertiliser to restore life to the Earth. Somehow, despite a world-wide nuclear holocaust, the background radiation has dropped back to pre-war levels within a year.
Turlough, baffled, returns to his ship, only to be surrounded by Gardsormr vessels -- and the leader of the Gardsormr emerges, to be revealed to be an elderly Turlough from an alternate timeline. The elder Turlough explains that the Gardsormr are in fact Trions who pretended to be aliens in order to ally themselves with the mad Rehctaht and undermine her authority from within. They continued the charade when it became clear that the New Regime had been infiltrated by Rehctaht's followers -- and that Rehctaht herself was not dead, but had gone into hiding. Somehow, it is she who is responsible for the destruction of Earth, Trion and New Trion, and it is up to Turlough to find and stop her. Turlough finally puts the pieces together and realizes that Juras is Rehctaht; the mad dictator implanted her own mind inside Juras' body, burying her own conscious thought processes beneath a charade of Juras and using the post-hypnotic spike to trigger her awakening whenever her plans were threatened. She has been leading Turlough on in order to find an answer to the scientific puzzle she was never able to solve; according to all of her scientists' best efforts, any Unified Field Theory should prove that the universal gravity constant is decreasing over time. Since it is not, Rehctaht has concluded that something is stopping it -- and if she can find the source of the effect, and control it, she will achieve supreme universal power. Rehctaht, realizing she has been exposed, attacks Turlough, who is forced to kill her although it still feels as though he is killing Juras Maateh.
To save both Trion and Earth, Turlough takes the ARTEMIS drive back to Trion during the last days of the Rehctaht regime, planning to offer her both the ARTEMIS drive and a Unified Field Theory in order to get close enough to kill her. He claims to have seen the error of his ways and presents her with his conclusions, but Rehctaht is so impressed by his work that she decides to possess his body instead of Juras'. Turlough is placed in an impossible maze where logic turns to panic as it slowly becomes apparent to him that there is no way out; once he is sufficiently disoriented, he is taken to the medical bay, where Rehctaht implants her mind within his. At this moment, the Magician arrives, kidnaps Turlough and uses his TARDIS to strand Turlough in the base of the Giant's Drop. Presented with another scenario from which it is impossible to escape, Turlough begins to realize that a voice is screaming at him in the back of his head; but this time it isn't his own, and he is able to resist it and drive it out, killing Rehctaht forever.
Once they are sure that Turlough is free of Rehctaht's influence, the Magician and the Slot return to rescue him. The Slot explains that his own people, the Laima, evolved beyond the need for the physical Universe millions of years ago; however, their passing to the next plane of existence was hampered by the decrease of the Universal gravity contant, and they therefore built an artefact to stop this and placed it on the then-uninhabited Earth. This is why Juras' gravity wave calculations came out wrong when she tried to take readings on Earth, and why the people of Earth erroneously believe that it is impossible to travel faster than the speed of light. With the Slot's help, the Magician pilots his TARDIS into an alternate timeline where the possessed Turlough fell to his death in the Giant's Drop while struggling to expel Rehctaht from his mind. Turlough takes his dead self's place in this alternate timeline, where, with Rehctaht well and truly gone, he is reunited with Juras -- the real Juras -- and prepares to introduce the ARTEMIS machine to his people.
Source: Cameron Dixon