Scattered throughout Time are certain events which will change the history of the Universe. A man who can move objects without touching them learns the truth of his powers from another man who claims to have a solution. Several million years in the past, the ghostlike figure of a man in furs watches the dinosaurs walk, unaware that he is being observed by a dinosaur which is trying to copy his bipedal stance. And in 1938, the Doctor -- stuck on Earth with no memory of his past, and awaiting his appointment with a man named Fitz in the year 2001 -- finds, in a small used book store, a journal of an expedition to Siberia, written by one Fitz Kriener. The future may hinge upon whether he decides to buy the book or not...

Many years later, from the Doctor’s perspective, Fitz bids farewell to his friends. During a recent adventure in Victorian England, he has befriended a young palaeontologist named George Williamson -- and, feeling that he needs to do something with his life rather than being the Doctor’s sidekick, he decides to join George’s expedition to Siberia. The Doctor allows Fitz to make his own decision and go, but when he and Anji return to the TARDIS, he’s forced to admit that he always knew how Fitz would leave him. Anji, who has already decided that she’s had enough, demands to be taken home -- and this time, the Doctor succeeds, missing his target by only three weeks. Anji soon settles back into her old life, but the absence of Dave Young serves as a constant reminder of her adventures, and eventually she decides to look up the fate of Fitz’s expedition. She finds only that the team vanished without trace, shortly before an enormous explosion which was heard as far away as Moscow. Anji must accept that she will never know what happened to Fitz, one way or the other.

In St. Petersburg, Fitz and George are surprised to meet the Doctor, whose coat sleeve is torn and who has come to ask a favour of Fitz. After the Doctor leaves, George and Fitz discover that the expedition leader, Paul Anderton, has fallen ill -- and George is disturbed to learn that their sponsors have replaced him with Hanson Galloway. Galloway, all bluster and bluff, soon alienates the other three members of the expedition -- Gerhart Graul, Peter Caversham, and St. John Price -- but George’s hatred of him goes deeper. Galloway built his reputation on research stolen from George’s mentor, Edward Parton, and when Parton was unable to prove this, he was forced to resign his position and committed suicide.

In 21st-century America, the mysterious Control interrogates Captain Andrew Jenkins, the pilot of a USAF spy plane which experienced something strange while flying low over Siberia. The plane and its crew are suffering from a bizarre time-delay, but the gap between real time and their perceptions is narrowing. Before the effect wears off completely, CIA scientists detect “chronic radiation” coming from the subjects, and Dr Ostrander is able to analyse it and build a machine capable of detecting anyone who has travelled through Time.

Englishwoman Miriam Dewes is accepted to a post at the Naryshkin Institute, a research station built into an old, abandoned castle in Siberia, where an international team of scientists is trying to create an artificial black hole. Their British sponsor, who seems desperate for the work to be completed, has suggested using ice to slow down the speed of light, and claims to be on the trail of a potential breakthrough. The castle appears to be haunted by the ghostlike figure of a man in furs, which the scientists interpret as an indication that their work will succeed; but nobody realizes that Miriam is secretly transmitting their research to her real employers, who warn her to expect visitors...

Galloway’s expedition approaches an old, abandoned castle, but their guide Chedakin refuses to proceed, claiming that there is bad magic ahead in a place where the worlds meet and where something he cannot explain is frozen in the ice. Galloway lashes out at him and at Price for failing to hammer tent pegs into the permafrost, and he and Fitz nearly come to blows before Caversham defuses the situation. That night, however, everyone in the camp hears a scream, and Fitz is the second to arrive at Galloway’s tent; George is already staring at the body and the tent peg which has crushed Galloway’s skull. The others accuse Fitz of the murder at first, and George confesses that he didn’t check whether Fitz came here directly from his tent. The others eventually decide not to press the matter, as there’s nothing they can do out here -- but Fitz fears that the real killer may try to dispose of him in a convenient “accident”.

Now travelling alone, the Doctor visits the British Museum, where the few surviving pages from Fitz’s journal are on display. There, he meets an American investor named Lionel Correll, who tells him that the rest of the journal has been found and is being sold at auction. Intrigued, the Doctor asks Lionel to save him a seat, and Lionel watches, amused, as the Doctor tricks the auctioneer into glancing at the seller, an elderly lady in the back row. The Doctor and Lionel drive up the price to see who’s interested in buying the journal, but the eventual high bidder is only an agent acting for someone else. Lionel asks around on the Doctor’s behalf and learns that the elderly woman is apparently the Grand Duchess Alice Romanov, the grand-daughter of Tsar Nicholas II -- which the Doctor finds quite odd, since he knows for a fact that the man she claims was her father was shot to death while only 13 years old.

Bidding au revoir to Lionel, the Doctor follows the Grand Duchess to Lakeside Manor Retirement Home, where, he loses track of her; most of the home’s residents are on an outing, and the only people present are the nurse, a young blonde visitor, and an old woman who looks very little like the Grand Duchess. Unsatisfied, the Doctor returns to the auction house, where the police have been called in following a disappearance and the discovery of an emaciated dead body. The dead man appears to have been crushed to death, the entire room has been twisted subtly out of shape, and on the floor the Doctor finds a smooth black pebble which he is unable to pick up -- just like one described in Fitz’s journal. Under the circumstances, the auctioneer, Mister Gilbertson, reluctantly breaks confidentiality and admits that the journal was purchased by the reclusive millionaire Maxwell Curtis.

Eighteen months after her return, Anji’s superior, Larry Withers, somewhat nervously introduces her to an American client, Alexander Hartford, who has specifically requested that she join his team as they assess work being conducted in Siberia. Anji is surprised when their plane takes off from a military airfield, and notes that his team behaves more like soldiers than financial analysts. As they near their destination, she becomes more suspicious, searches the plane, and finds ammunition and weapons in the cargo bay. Hartford and his “personal assistant” Thorpe catch her and try to force her back into the passenger area at gunpoint, but she locks herself in the cargo bay, tosses a parachute out through the doors, and hides. Concluding from his GPS tracker that she’s now directly beneath the plane, Hartford orders his team to jump, intending to pick her up on the way to the target.

Anji waits for the others to go, and heads for the cockpit, intending to somehow overpower the pilot and co-pilot -- only to find that the plane is expendable, and that they too have jumped, leaving her stuck on the plane as it heads for a mountain range near an old, abandoned castle. At the last moment, she discovers that the pilot’s seat can eject; she thus escapes before the plane crashes, but ends up stuck outside in Siberia without suitable clothing. She tries to crawl towards the castle for shelter, but Hartford and his team find her first. This time, Hartford reveals the truth; his team is here for the time-travel equipment being developed at the Naryshkin Institute. Since the Institute is the only building in the area where Jenkins’ plane entered time-lag, the CIA has concluded that the experiments being conducted there are responsible -- and since their chronic radiation detector located Anji, Hartford assumes that the scientists at the Institute have sent her through Time.

Back in 1894, Fitz and his companions bury Galloway under a cairn and continue on towards the castle. In a nearby valley, however, they encounter a bizarre hole in the air through which they can see a grassy veldt. George tosses a rock through the “window”, breaking an invisible barrier, releasing a blast of energy and causing something on the other side to begin howling. The expedition members retreat, but the creatures from the other side -- eight-foot dinosaurs which walk on two legs -- pass through the window and attack, killing Graul. Pursued by the saurians, the survivors flee to the castle and barricade themselves in. Caversham stands guard on the entrance while the others look for an easily defended room, and in case he doesn’t make it, he gives Fitz the grenade which he intended to use to blast fossils out of the rock.

The Doctor tracks down Maxwell Curtis, a recluse who made his fortune as a stage magician known as “the Great Attractor” and who now funds research into the origins of the Universe. The Doctor breaks into his home, where he finds all of the furniture has been reinforced and bolted down to floors which don’t creak under his weight. Meanwhile, Curtis reads the journal which he bought at the auction, finds the information he needs, and contacts Naryshkin and orders him to send a team to investigate. Naryshkin does so, and finds just what Curtis expected -- and much more besides. As Naryshkin reports to Curtis, however, Hartford’s team attacks and Curtis loses contact with the Institute.

Disturbed, Curtis orders his “servant” Holiday to contact the Grand Duchess again, hoping that she may have a deeper insight into the journal’s contents. Holiday reluctantly does so, and then catches the Doctor and turns him over to Curtis. Curtis doesn’t believe what the Doctor has to say about the journal, even when the Doctor points out that it describes Galloway as being in the castle’s Great Hall when the monsters attacked. However, Curtis does see that the Doctor’s insight could prove valuable, and thus demands that he and the Grand Duchess accompany him to Siberia. The Doctor requests that his TARDIS be brought along as well. On the flight, while trying to provoke the Grand Duchess into admitting the truth about herself, the Doctor describes the “Schroedinger’s Cat” thought experiment. Holiday scoffs at the theory, which states that according to quantum theory, any event with multiple possible outcomes exists in an indeterminate state until an act of observation collapses the probabilities into certainty and causes the Universe, if necessary, to split into distinct quantum realities.

The peaceful scientists and Russian soldiers at the Naryshkin Institute are entirely unprepared for the Americans’ assault. Hartford forces the scientists into the castle’s Great Hall, where he orders them to hand over their time machine and all data pertaining to their time-travel experiments. The scientists have no idea what he’s talking about, and he begins to shoot them one by one until Anji, desperate to stop him, claims that they will send her back through Time but haven’t yet. Hartford accepts this enough to stop killing people for a while, though he and his team still have no hesitation about killing anyone who gets in their way -- such as Yuri Culmanov, who gets himself killed trying to explain that their real work is related to the theory that nascent atoms of dark matter are present within normal matter, awaiting a trigger to collapse into black holes.

Unable to detect chronic radiation in the Institute, Hartford concludes that Anji’s presence is swamping the detectors, and orders two of his team to take her away. British scientist Basil Flanaghan is forced to guide them to the ruins of a nearby village, and on the way he explains to the sympathetic Anji that his work is related to the wave/particle duality of light. When a single photon shines through a barrier with two slits, it creates interference patterns unless equipment is set up to detect which slit the photon “actually” passed through -- as though the one photon passes through both slits simultaneously until the act of observation forces it to choose which one it “really” passed through. Flanaghan then tries to distract the guards and rescue Anji, but an exchange of gunfire results in the deaths of Flanaghan and the guards, and in the confusion Anji loses her coat and ends up lost. However, she is being observed by an SAS team, and when she passes out from exposure, Captain Nesbitt and Corporal Lansing inject something into her neck and carry her back to the Institute. Hartford and his team assume that Anji struggled back to the castle and collapsed after pressing the intercom.

Back in 1894, as Fitz and his friends barricade themselves inside the castle’s Great Hall, they hear a strange noise in the corridor outside. Caversham and George investigate, but Caversham vanishes after a gunshot is heard, and Fitz finds only a strange, smooth black pebble which he is unable to pick up from the floor. The saurians then break in, and Fitz inadvertently finds a secret passage while pulling down a tapestry from the wall to defend himself. Realizing that the saurians will follow and kill them all, Price stays in the Great Hall, giving his life to hold off the saurians while Fitz and George escape. The passageway leads to another room which overlooks the courtyard, where George tries to get some sleep while Fitz updates his journal, describing everything that’s happened so far.

Curtis’ plane arrives at the Naryshkin Institute, where Hartford lets them land but shoots the pilot and co-pilot. The Doctor is infuriated, even more so when Thorpe shoots young scientist Penny Ashworth simply for leaving her room without permission -- but when he sees the ghost of the man in furs, he realizes that things are more dangerous than he’d suspected. Curtis is appalled to learn that the real experiments have been suspended; he seems quite ill now, and when shown to his room he sits on a chair so heavily that it breaks. The Duchess is shown to a room overlooking the courtyard, while the Doctor is allowed to examine Anji, who is recovering in the Great Hall. Once they’re alone, they compare notes, and the Doctor reveals that he’s recognized the ghost as George Williamson.

Morning in 1894. Fitz and George try to slip out of the castle, but the saurians come out of hiding and pursue them. As they flee, loose papers fall out of Fitz’s journal and scatter; these will later be displayed in the British Museum. Fitz and George take shelter in a nearby cave, where they find flames flickering within the ice -- and find an ice-statue replica of the TARDIS. As the saurians approach, Fitz thinks back over the events which led him here, and it finally occurs to him that George, the only man with a motive to kill Galloway, was standing over the body when Fitz arrived. The saurians enter the cave, and with no time left to make a decision, Fitz throws Caversham’s grenade at the ice wall. He pushes George to safety and rushes to the ice-TARDIS for shelter, but the explosion is more devastating than he’d imagined and the blast is heard as far away as Moscow...

The Doctor uses the secret passage described in Fitz’s journal to get from the Great Hall to the Grand Duchess’ room without Hartford noticing, intending to find the ice cave where Fitz made his final stand. Anji insists upon accompanying him, as she is the only one whom Hartford won’t kill on sight. As they slip out of the castle, the Doctor describes its layout and the position of Hartford’s team -- for the benefit of the people who planted the tiny camera lens he can see in Anji’s neck. At the ice cave, the Doctor and Anji, like Naryshkin before them, find George Williamson’s body frozen in the ice. The Doctor chips him free, and George wakes, disoriented -- but he’s still as insubstantial as a ghost, and there’s still an image of him in the ice. Before returning to the Institute, George is forced to admit that Fitz is dead, and the Doctor admits that he’s known this ever since he saw Fitz’s handwriting in Spain and realized that it matched the handwriting in the journal he’d purchased in 1938.

Hartford’s mission is going very badly; he has been unable to find the Institute’s time-travel experiments, and now Curtis Maxwell has gone missing, along with many of Hartford’s soldiers. Control orders Hartford to cut his losses and get out, but as Hartford’s remaining men plant explosive charges about the Institute, the Doctor arrives with the ghostly George and reveals that the “time machine” is a natural phenomenon, which is why the scientists didn’t know what he was talking about. Hartford has Thorpe and Private Jonas escort the Doctor, Anji, the Grand Duchess and the TARDIS to the ice cave so the Doctor can prove his point, and on the way, they find another smooth, immovable black pebble where one of Hartford’s soldiers was supposed to be standing guard. The Grand Duchess finally admits that, as the Doctor suspected, the journal she sold to Curtis was a fake supplied by a business associate who wished to ensure that Curtis found the cave.

When Thorpe and Jonas slide the TARDIS into the cave, it comes to rest exactly where George and Fitz saw the ice-TARDIS in 1894. The Doctor flips a coin to decide whether or not to move the TARDIS, and when he does it leaves a perfect ice replica of itself behind. The Doctor and the others then detour to examine the “window” into the saurians’ world, but Jonas panics when he sees a saurian on the other side and tries to shoot it, breaking the “surface tension” between the worlds just as George’s rock did in 1894. One of the saurians leaps through, kills Jonas and attacks the Doctor, tearing his coat and driving the others away. When they return without the Doctor, Hartford loses his temper, shoots Miriam Dewes dead, and threatens to kill every remaining scientist if they don’t hand over the time machine within thirty seconds.

Nesbitt’s team drives back the saurian and rescues the Doctor, but are surprised when their medical equipment detect that the Doctor has just finished growing a second heart. The Doctor questions them and learns that British scientists have detected powerful gravity waves coming from the Institute; Nesbitt’s team was sent in to protect the scientists from whomever might try to steal their work, but their rules of engagement didn’t take into account an attack by US special forces. Miriam Dewes was their agent on the inside, but now she’s dead -- and the gravity readings went off the scale shortly after the Doctor’s party arrived. The Doctor warns them that Hartford is just a distraction from the real danger. The “frozen flame” in the ice is light from an o-region, an area of space so distant that its light has yet to reach this part of the Universe. The o-region’s light moves more slowly than light from this area of space, which means that the o-region’s inhabitants could see us before we could see them. But this too is just a sideline; the real danger lies in the nature of quantum theory and alternate timelines, and the implications of what the Doctor has seen so far are terrifying, especially for George Williamson.

Nesbitt agrees to take back the Institute, using the information from the camera they planted in Anji’s neck. However, most of the American soldiers have disappeared, including Thorpe -- and the Doctor finds out why when he and Hartford are attacked by Curtis, who has killed the others by crushing their bodies down to smooth, heavy black pebbles and absorbing their life forces. Hartford, realizing that Curtis poses a threat to the entire world and that only the Doctor can stop him, gives his life to slow him down while the Doctor retreats. Curtis follows him to the Great Hall, but the Doctor and Holiday talk him down, helping him to hold onto his personality for just a while longer. As Yuri Culmanov suggested earlier, the Universe is full of nascent atoms of dark matter waiting to collapse -- and Curtis, the host of these elemental forces, is becoming a black hole. When the forces become too much for him to hold back on his own he reluctantly commits murder, using his victim’s life force to hold back the Darkness for a while longer. Holiday helped him to set up the Institute and fund research into a way to slow the speed of light and travel back in time, thus reversing what is happening to him. He has already tested out the time envelope by going back to 1894, killing Caversham while there, but he was too frightened to go any further. However, he now has no choice; he can’t hold back the Darkness any longer, and must go back to a time before the Big Bang, before the dark matter within him was created, thus expunging it from his body and saving himself and the world.

The Doctor, unable to stop Curtis from leaving the Institute, demands that the impostors in the Great Hall shed their disguises. The Grand Duchess is thus revealed to be the young, blonde con woman Beatrix “Trix” MacMillan, whom Holiday hired as a front to supply the fake journal to Curtis -- and Holiday is revealed to be Sabbath. Sabbath tries to hold he Doctor at gunpoint to prevent him from interfering, but Nesbitt’s men arrive and shoot the gun out of his hand; the Doctor offers him a handkerchief to bind the wound, and later takes it back.

Sabbath reveals that when the ice exploded in 1894, George was caught between two intersecting shafts of slow light, giving him the ability to walk through the dimension of Time. George admits that he walked back to see the dinosaurs, and the Doctor theorizes that this act may have created the alternate history on the other side of the “window”, which is partly why the two realities are close to overlapping. His walk also created a time envelope which Sabbath has extended back before Event One, to Time Zero. As always, Sabbath intends to collapse all alternate timelines into one manageable history. Curtis believes that the journey through Time will cure him, but in fact he will collapse into a black hole and release the pent-up energy within himself before Event One. This defining event at Time Zero will shape all that follows, destroying all alternate histories and leaving only one possible timeline.

At least, that’s Sabbath’s theory, but the Doctor insists that he’s wrong about the nature of the Universe. Every significant event remains indeterminate until the act of observation forces the Universe to “choose” the outcome. It does not split unless absolutely necessary, and even then, under ordinary circumstances it’s impossible -- or at least very, very difficult -- to move between alternate timelines. If Curtis reaches Time Zero, however, it will become impossible to choose between alternatives, for there will be no alternatives to be chosen. It seems that whoever or whatever “educated” Sabbath on the nature of Time is really only interested in eliminating the concept of free will...

To prove his point, the Doctor has Nesbitt’s men bring the ice-TARDIS into the Great Hall, and when he smashes it, Fitz falls out, still alive. Massive objects distort space and time, like a weight on a rubber sheet -- and the transforming Curtis is such a weight that his presence is causing alternate realities to intersect, becoming visible as ghosts or solid manifestations. The ice-TARDIS first appeared as a representation of the possibility that the real TARDIS might appear in the same place, and Fitz survived in an indeterminate state because nobody actually saw him die and the Doctor and Anji refused to accept that he was dead.

While recovering, Fitz notices the Doctor’s coat, torn where the saurian attacked, and mentions having seen the same damage in St. Petersburg. The Doctor, who hasn’t been to St. Petersburg yet, realizes that there may be a way to resolve matters -- though the price may be high. He takes a quick trip in the TARDIS and then returns, but before finishing his work he defuses Hartford’s explosives, and logically proves Sabbath wrong. The slow light was attracted to Earth by Curtis’ transformation, and in order for it to have arrived by now the transformation must have taken place in the past... which means that Curtis must have travelled back in Time before the transformation... but the time machine that he’s using to do so was created by the slow light. This paradox can only take place in one continuous timeline, which means that time travel alone does not split the Universe; history can incorporate paradoxes into itself, and will only split when there is no other alternative (as it were).

Echoes of images from the past begin to appear in the castle as the separate timelines begin to merge, positive evidence of Sabbath’s grave error. The Doctor takes Anji, Fitz and George to the ice cave to finish matters, while Nesbitt’s men keep Sabbath under guard in the Great Hall. Trix questions him further, intrigued by what she’s learned so far. Sabbath deduces that the Doctor intends to disperse the black hole’s energy in 1894, which could cause devastating side-effects. Fortunately for Sabbath, he has a model boat which has given him an idea for a backup plan -- and Trix offers to show him a way out if he tells her more about the Doctor.

Back in the ice cave, the Doctor reveals that George is ghostlike because his future remains indeterminate. He may have been touched by slow light and survived, opening the time envelope and enabling Curtis to reach Time Zero -- or he may have died in the ice cave, in which case Curtis is merely travelling along the slow light himself and the energy of the black hole will be dispersed when Fitz destroys the cave, which the Doctor has just asked him to do, back in St. Petersburg in 1894. Fitz is quite happy to see his murderous “friend” go to his death, but George admits that although he did kill Galloway he did it in self-defence. Galloway attacked him, fearing that George was going to expose him as a fraud, and George didn’t cover for Fitz because it honestly never occurred to him that the others would see Fitz as anything other than a good, decent man. George makes his decision and returns to 1894, where he steps out of the ice in front of his younger self. The younger George falters in shock, and is crushed by the falling ice when the grenade explodes. Thus, George never walks through Time, the envelope back to Time Zero is never created, and Curtis, trapped in the slow light beam, is destroyed when the grenade brings down the ice cave, releasing the latent energy of the black hole in a blast heard as far away as Moscow. The slow o-region light spills out over the landscape; most of it seeps into the Earth and is absorbed into the magma, creating a new elemental life form, while some scatters across local space-time, causing random side-effects such as the misphasing of Colonel Andrew Jenkins’ USAF spy plane.

Sabbath escapes from the Institute, either because Trix showed him the passageway or he saw it himself when echoes of the past appeared in the Great Hall. Trix offers to trade information to the Doctor in exchange for a ride in the TARDIS; he flatly refuses, but she tells him anyway that Sabbath spoke of “a race against infinity.” Anji accepts the Doctor’s offer of a lift back home to London, but as he unlocks the TARDIS he hears a strange noise behind the ship. He and his friends circle the TARDIS, find nothing, and dismiss the noise as unimportant.