Pierre Yves-Dudoin, Arthur Tyler III and Christine Holland were brilliant classmates and good friends until a love triangle developed between them. Pierre won Christine’s hand in marriage, and they produced a daughter, Pippa; however, they eventually divorced when Pierre became obsessed with his dream of becoming the first privately-funded man in space, and lost sight of his family. Christine retreated to relative obscurity, lecturing at a British university and raising her daughter, while Pierre and Arthur settled into a bitter rivalry, each striving to reach space before the other. Finally, on the 6th of February 2001, Pierre announces that he has succeeded; his Star Dart will be in space within a week. What nobody knows is that his team has been helped, by an alien who is just as desperate to leave the Earth as Pierre...

In Brussels, two assassins -- one French, one alien -- pursue a man named Menhira and gun him down before the eyes of two British tourists, stockbroker Anji Kapoor and her boyfriend, actor and sci-fi buff Dave Young. Dave tries to administer CPR, but as he does so he realises that the dying man has inhuman blood and two hearts. In the confusion Dave fails to notice Menhira slip a package into his pocket, and inject something from a syringe into his arm; he feels the sting but later assumes it to be an insect bite. The killers disguise themselves as paramedics and take the body away, but Dave is interviewed by the press, and his claim that the dead man had two hearts makes the news even in London. There, Fitz Kreiner sees the news report while window shopping in an electronics store. Compassion has just dropped him off, with instructions to meet the Doctor at a pub called St Louis’, but Fitz knows that it would be just like the Doctor to hang around on Earth for a century and then get himself killed in the wrong city two days before their scheduled meeting. Fitz thus visits Brussels to speak with Dave, and although he soon determines that the dead man was not in fact the Doctor, he’s still curious enough to investigate.

Dave finds the package in his pocket, telephones the number written on it, and is surprised to find himself speaking to Arthur Tyler III. Tyler sends his bodyguard, Marshall Spear, to meet Dave at the Atomium and collect the package. Anji wants no part of this, but Dave and Fitz, hoping for a reward, go to the Atomium empty-handed in order to discuss terms. Spear, however, threatens legal action if they don’t hand over the package, and, suitably cowed, they agree to return to the hotel to fetch it. But the assassins have been keeping Dave under observation ever since realising that the package wasn’t on their victim’s body. One of the killers goes to the hotel room for the package while the other attempts to kill Fitz and Dave; he fails, and falls several metres from a catwalk... and then gets up and runs away. Dave and Fitz return to the hotel to warn Anji of the danger, and their arrival frightens off the other killer, who drops a gun as he flees. When Fitz takes a close look at the gun, he realises that it wasn’t manufactured on Earth. Dave finally agrees to return the package to Tyler with no more fuss -- but the killers are waiting outside, and he is kidnapped as he leaves the hotel. Anji wants to go to the police, but somebody steals the alien gun, leaving her with no real evidence of wrongdoing. Anji thus accepts Fitz’s suggestion that they return to London, to meet a friend of his who might be able to help her...

It’s the 8th of February, 2001. Fitz and Anji visit the St Louis’ pub, which is in fact owned by the Doctor; since he was unable to locate the correct St Louis’, he made one himself, to bring Fitz to him. At last the two friends are reunited, but the Doctor still doesn’t quite remember how he and Fitz know each other. Fitz is unsettled by the Doctor’s apparent memory loss, and by the fact that the police box in the Doctor’s office is smaller on the inside than the outside. Anji is beginning to wonder what she’s gotten herself into, but when the Doctor switches on the charm he’s oddly comforting, and she tells him all about Dave’s disappearance. The Doctor agrees to help. He and Anji will investigate Tyler and the mysterious package, while Fitz returns to Brussels to see if Tyler’s space-race rival Dudoin is involved in any way. What none of them yet realise is that an alien invasion fleet is already in the solar system. The Kulan have been waiting for word from their spearhead for four years, and they’re getting impatient...

While checking out Dudoin’s base of operations in Brussels, Fitz meets a man named Fisher who apparently believes that Dudoin’s company, ITI, has made alien contact. In fact, Fisher is the man who stole the gun from Anji’s hotel room; he is a CIA agent who reports directly to the man called Control, and Control wants this problem solved without getting UNIT involved. Fisher and Fitz break into the ITI building, where they find an alien named Sa’Motta tending to the dying Dave. Sa’Motta turns out to be friendly, although he did come to Earth as part of an invasion spearhead. The spearhead crashed four years ago and has been cut off from the fleet ever since, preventing their leader, Fray’kon, from reporting that the Earth is a suitable target for invasion. Realising that Dudoin is too obsessed to question the Kulan’s agenda, Frya’kon has been helping him to finish his work in order to accompany him into space and get back to the fleet. Some of the other Kulan, however, simply want to return home, and they’ve been helping Tyler in the hope of getting to the fleet first and recommending that the invasion be called off. Since human minds cannot operate the Kulan’s biotechnology safely, the Kulan have been trying to develop a human-Kulan hybrid in order to do so; Menhira was trying to smuggle the specifications to Tyler, and while dying he injected the biodata sample into Dave’s body to preserve it. However, Dave’s body is rejecting the mutant genes. Fisher agrees to help Dave and Sa’Motta to escape, planning to turn them over to Control, but as they try to flee they are spotted by security guards. Fitz and Sa’Motta escape, but Dave is recaptured and Fisher killed. Control, knowing only that Fisher is dead and that Fitz and Sa’Motta were in the vicinity at the time, orders an Offensive Action Team to bring in Fitz and Sa’Motta... not necessarily in one piece each.

Anji accompanies the Doctor to Tyler’s base, where she’s surprised when the security guards wave them through -- and horrified when the Doctor admits that he used her personal computer to hack into Tyler’s network and grant himself an appointment. They break into an empty office to access the company’s secure servers, where the Doctor finds evidence that the Planet Hopper has been modified with alien technology. Before he can learn anything more, an alarm goes off in the testing area; Dudoin has sabotaged the Planet Hopper with a Kulan computer virus, and Tyler is trapped inside as a fire breaks out. The Doctor arrives just in time to save Tyler’s life, and transfers the virus onto a recordable DVD for later study. The grateful Tyler admits all, and the Doctor offers to help him stop Dudoin and Fray’kon -- but before they can do anything, a post-dated command planted by the Kulan virus causes the Planet Hopper to explode.

Tyler can’t proceed without adapting the Kulan’s biotechnology to work with human minds, and as Menhira’s data was intercepted he has to turn to Christine Holland for help. He’s been trying to speak to her for some time, but their friendship ended bitterly when she married Dudoin and she’s been refusing to return his calls. The Doctor offers to speak to her, but he’s too late; Dudoin has already kidnapped Pippa in order to lure Christine to Brussels. Although furious with him for using their daughter as leverage, Christine is fascinated when he introduces her to a real alien, and agrees to put her old skills to work to save Dave Young’s life. She does so, and Dudoin, who cares for nothing but the fulfillment of his lifelong dream, announces that he will launch the ship immediately without waiting to test it. Christine becomes suspicious when she sees Fray’kon tampering with the ship’s controls, but Dudoin refuses to listen to her and has her locked up to prevent her from interfering with the launch.

Fitz and Sa’Motta attempt to return to London, but realise that they’re being followed and flee into the streets of Brussels. Sa’Motta gets away, but Fitz is captured by Control’s team and taken for questioning. Sa’Motta knocks out the agent following him, steals her belongings and traces the secret CIA base where Fitz is being interrogated. Realising that Fitz knows nothing important, Control has his team fit a tracer to Fitz’s jacket and allow Sa’Motta to “rescue” him; the two of them will lead his team to the other aliens. Fitz is unaware of this, as he has weightier issues on his mind; while thinking about all he’s been through recently, he has realised that his memory of the details is growing cloudy. Did the Doctor really destroy his own homeworld or did Fitz just imagine it? Meanwhile, the rest of the Kulan fleet moves past Mars, into position. The fleet leader, Koy’Guin, is Fray’kon’s cousin, and he refuses to have spent all this time here for nothing. Earth will fall...

While trying to contact Christine, the Doctor discovers that Pippa is being held captive, and helps her to escape. Dudoin then announces the launch of the Star Dart, and the Doctor and Tyler realise that they must stop him; if Fray’kon gets into space first, the Earth will be invaded within the day. The Doctor, Tyler, Spear and Anji thus attempt to break into Dudoin’s launch site; fortunately, the guards are distracted by Fitz and Sa’Motta trying to break in from the other side. The Doctor and his friends find and rescue Christine, who confirms that the Kulan biotechnology is controlled telepathically. The Doctor thus attempts to link himself to the Kulan systems to shut down the Star Dart by remote control, but the pain is too much for him and he passes out. As he does so, he boosts the oxygen production in the Star Dart in the hope that the fail-safe systems will prevent the launch; however, no such system exists, and the overabundance of oxygen causes the Kulan to pass out. Fray’kon realises what is happening and escapes, while Dudoin struggles with the controls in the Star Dart, refusing to abandon his ship. Believing that the Doctor has failed, Tyler uses the DVD to sabotage the Star Dart with the same virus Dudoin used against him earlier, causing a short-circuit which ignites the oxygen-rich atmosphere in the cabin. Dudoin and the Kulan perish in flames.

Subdued by the outcome of their sabotage attempt, the Doctor and his friends return to Britain to get the Planet Hopper ready for launch, so Sa’Motta can return to the fleet and recommend their withdrawal. As soon as the Hopper is ready to launch, Tyler, Spear, Fitz and Sa’Motta board; however, the Doctor lags behind to check on Dave’s condition. Dave is recovering now, thanks to Christine, but according to his medical records Dave already had a small amount of Kulan DNA in his makeup even before Menhira injected the sample biodata into him. This may indicate that humanity is itself an offshoot of the Kulan race, which would give them another reason to call off the invasion. As the Doctor sets off to tell Sa’Motta, Dave and Anji get into a fight; Anji has been so worried about Dave that she’s infuriated when he says he wishes he was going with the others. When she storms out on him, he realises rather belatedly that he loves her more than the adventure, and follows to apologise... only to run into Fray’kon, who has followed them from Belgium.

Control’s team storms the compound to prevent the Hopper from taking off with Sa’Motta aboard, but Tyler hears the commotion and switches off the communications system so he won’t be forced to stop the launch. Meanwhile, Fray’kon takes Anji hostage, forces the Doctor to hand over his spacesuit, and forces Dave to drive him to the launch platform -- where he murders Dave before Anji’s eyes and boards the Hopper. Anji must flee as the Hopper takes off, incinerating Dave’s body. The CIA team is forced to withdraw, their mission a failure, but with the comms system down, the Doctor can’t warn Tyler that Fray’kon has smuggled himself aboard -- and before Tyler can reactivate it, Fray’kon cuts the comms circuitry and kills Spear and Sa’Motta. Fray’kon then orders Tyler to take him to the fleet, but the fleet comes to them instead, as Koy’Guin’s flagship spots the Planet Hopper and tractors it aboard. Tyler and Fitz are thrown into a cell on the flagship, while Fray’kon reports to the Council of Three that the treacherous human race murdered the rest of the spearhead, and must be wiped out...

The Doctor and Anji return to London, where the Doctor wanders back to his pub, despairing over his inability to reach Fitz. Anji, though devastated by Dave’s death, refuses to succumb to despair and forces the Doctor to snap himself out of it. If the Doctor is everything Fitz said he was, there must be some other way for him to get to the fleet; what about this time machine Fitz kept going on about? As if on cue, the Doctor realises that the police box in his office is humming, as though it’s alive... and when he opens it up, for the first time in a century, the inside is larger than the outside. Anji watches in amazement as a huge control room shapes itself into existence out of the empty void within. The Doctor is delighted... at last, at long last, he’s back where he belongs. This is his ship; this is his TARDIS.

Anji must forcibly restrain the Doctor from setting off to explore the TARDIS, reminding him that there’s an invasion to deal with first. The Doctor crosses his fingers and tries to pilot his TARDIS to the Planet Hopper, but instead he ends up on the Kulan flagship. He sets off to explore, telling Anji to stay where she is, but she follows him, and sees the Kulan capture him and throw him into the cells with Fitz. The Council has Tyler brought before them to answer the charges against his species, while Fray’kon, certain of success, ejects the Planet Hopper and has it used for target practice. Anji watches from hiding as Fray’kon fires the first shot himself. The Council then order Fray’kon to bring the Doctor to them, and Anji follows him to the cells and breaks Fitz out as soon as the coast is clear. She and Fitz then return to the weapons room, where Anji knocks out the guard and tries to bluff the fleet into retreating by claiming that humans have taken over the flagship. Trying to remember what Fray’kon did, she fires at the nearest ship to reinforce her bluff; unfortunately, she doesn’t understand the controls as well as she thinks, and unleashes a barrage of missiles which destroy not only the ship she aimed at, but a number of neighbouring ships as well. Worse, the rival Kulan families have never trusted each other, and when destroyed by one of their own, the ships release primed mines which sweep through the fleet, causing further destruction.

As the fleet tears itself apart, the Council panics and orders Fray’kon to execute the Doctor and Tyler. Anji and Fitz arrive to see the Doctor and Tyler struggling with their captors, and in the confusion, Fray’kon apparently slips and hits his head against the wall. The Doctor and his friends try to flee back to the TARDIS, but the disoriented Fray’kon pursues them; however, Tyler reveals that he’s already dying of a brain tumour, which was why he was so desperate to get into space in the first place. He thus lures Fray’kon into an airlock and ejects both himself and Fray’kon into space, giving the Doctor, Fitz and Anji the opportunity to board the TARDIS and escape. The Doctor dematerialises, and he, Anji and Fitz watch on the scanner as the Kulan fleet blows itself apart. Sa’Motta’s existence proved that some of the Kulan might have listened to the Doctor’s attempts to talk peace, but they’ll never know now.