On his trip moving from London to a town in the north of England to take a new job at a web development firm, Will comes down with a severe flu. After finally making it to his new house (and discovering that his belongings never arrived because the removal van got in a pile-up), he barely has enough strength to wander up his street searching for help, as he has no idea where he might find a chemist or an A&E. He is finally taken in by a kind neighbour named David, a self-described inventor, who nurses him back to health and offers him a place to stay while he waits for his furnishings.
David seems a bit strange (on the first day, he goes out to purchase things to furnish his otherwise empty house, simply to make Will more comfortable during his stay, for example) but is generous and friendly, and they become fast friends. David admits that he’s a bit of a loner, unsure of himself after a serious accident two years before had left him with a change of personality and outlook. He’s currently living in his brother’s house, trying to re-establish his life. To help him boost his social confidence, Will invites David to go out to the pub with his new co-workers, where he thrives among the friendly, open people, though he is unable to recognise that one of the women there was showing a very obvious interest in him.
Over the next few months, Will establishes his life in his new home. He’s very much a gamer and a tech geek, and finds good friends in his co-workers, especially Ben, with whom he shares a number of common interests. Ben is a history buff, always eager to explore historical sites with Will, and David joins them on most of their outings.
On the day of the company picnic, to which Will has invited David, Will arrives at David’s house whilst the man is away, and after he lets himself in with the key David had insisted he keep, he is tempted to explore the upstairs of the house, where David had told him he does his work. He finds both rooms empty except for a bed, a wardrobe, and a strange metal cylinder with a door that he is unable to open. There is no indication of any invention being done in the house, and Will begins to wonder what his friend is hiding. David arrives home, having gotten delayed whilst returning from the Tesco, and the two head for the picnic.
After lunch, whilst Will is playing football with the rest of his company, an alien spacecraft appears overhead and begins to descend on the park. As everyone flees, Will tries to convince David to run as well, but he insists that they stay nearby to watch the landing. Once the spaceship lands and humanoid rhino soldiers march out, David identifies them as Judoon and translates what they’re saying for Will: they’re looking for something, though they never mention what it is. The two then retreat back to David’s house, with David all the while trying to figure out what to do about the aliens.
Once in, Will, angry, scared, and suspicious, demands to know who David is. The man admits that he’s an alien, a Time Lord, who was once a human copy of another Time Lord called the Doctor, and is living in a house lent to him by the Doctor whilst he builds his spaceship. Will is appalled that David did not confide in him - and obviously still hadn’t intended to do so even after the spaceship appeared - yet expected him to follow him and help him deal with the aliens, and this forces David to realise that he hadn’t trusted his friend and had endangered him by simply being there. Apologising, David urges Will to run, to escape the town before the Judoon find him, and retreats upstairs.
Betrayed by his friend, Will is about to leave, but he realises that no one else knows these aliens and has any chance of rectifying the situation, so he follows David upstairs to offer his assistance. Once up there, he finds the door to the metal cylinder open, and looking in, finds that the chamber inside it is far larger than the cylinder itself: this is the spaceship that the alien is building. Amidst the piles of materials and tools strewn everywhere, David is standing, hunched over the central table, tired and discouraged, with the pendant he always wears dangling from his hand. Will offers his assistance, which David accepts gladly. David explains that they’re on their own, because, though the Doctor normally comes to Earth’s assistance and deals with situations like this as a matter of course, he has not appeared this time and David cannot call him for help because if a previous incarnation answers the summons, it would cause a dangerous paradox. The pendant he wears is a neural inhibitor, which shuts down his psychic abilities so that the Doctor cannot sense him if he happens to visit the planet.
David approaches the Judoon to find out what they want and learns that they’re searching for a total fission device, a shoebox-sized weapon that disintegrates planets. When the rhinos refuse his assistance in searching for it because he is not a human and therefore not involved in the case, he officially takes responsibility for the town and planet; this means that he is allowed to search for the device, but if the Judoon decide on a verdict of guilty of possessing a forbidden weapon, he will be executed. David and Will then search every possible place they can think of - anything new in town; weird, suspicious people and places; anything high-tech - but come up empty, and when the Judoon summon David for sentencing, Will is left to continue the search alone. After a few more failed attempts, Will remembers an archaeological dig that Ben told them about, back on that first pub outing. He goes there and finds that yes, the scientists had unearthed the device.
Returning with it to the spaceship, the Judoon confiscate the device and are about to leave without any further problems when David realises the device is armed, primed to go off either when the Judoon are in space or after they return to the Shadow Proclamation. After disarming the device, he admits to Will that he didn’t actually know how to do it and was basically working the instructions backwards. Appalled that he had risked disintegrating the Earth in order to attempt to save the aliens that had threatened the town, Will tells him off for the second time in the day, and David stalks off, saying that he refuses to favor one species over another.
After a week of no contact between the two, David visits Will to say goodbye and tell him that he’s leaving the Earth to spend the next couple of years building his spaceship in the time vortex, where he’s no threat to anyone else. Will realises that David is blaming himself for everything and punishing himself for it, and that if he hadn’t been there, the Judoon situation would have turned out very differently. He explains to David that whilst he might be an alien, he still belongs on Earth, that he fits in not because of his species, but because of himself, and that he has to learn to trust his friends. He also admits that he reacted poorly to David’s revelations out of fear of the man himself. For his part, David explains that as a Time Lord, he has a greater responsibility to the universe and cannot always act in the best interests of Earth. The two friends come to a mutual understanding, and David confides in Will, telling him his whole story.