Angel, a young gay commercial photographer, has had a bad night. He's been out with Martes, a hip, supposedly heterosexual creative director at a big ad agency who has been hiring Angel frequently. Angel is drawn to Martes and knows the feeling's mutual, but when he tries to take their flirtation to the next level, Martes shoots him down defensively, claiming he's not gay. Drunk, bereft, and exhausted, Angel makes his way home, only to find a group of teenage thugs attacking a small defenseless creature in the parking lot of his apartment building. He comes to its defense and finds that it is a juvenile troll. It appears sickly, wounded, and totally helpless. So Angel takes it inside for protection.
In the morning, he almost thinks he dreamed it. But there it is, and so, head splitting with a hangover, Angel goes to the internet to try to figure out what a troll might eat. He finds a great deal of folklore-about how trolls are truly man's evil doppelganger, or occult forest creatures. He calls a veterinarian ex-boyfriend, whom he jilted for Martes, and who bitterly hangs up on him after telling him that a troll's system is somewhat similar to wild cats. Desperate, he goes to his downstairs neighbor's, where a Filipina mail-order bride, left alone for the day by her brutish husband, answers the door. No, she can't lend him any cat food, but she does offer him some frozen fish. The troll refuses to eat. Eventually, by dint of intense research, Angel begins feeding the troll mice and guinea pigs from several different pet stores, afraid to return to any too soon. It makes a messy kill that disgusts him, but soon its health improves. Angel begins to get quite attached to his new pet. Reading a fairytale about a fairy-girl and a troll (who stand in for idealistic illusions versus earth pessimism), Angel decides to name his charge Pessi, after the one in the book.
Meanwhile Martes has offered Angel a photography commission that is sure to be lucrative-to create images for a campaign for a new brand of trendy, high-end blue jeans called Stalkers, that will be promoted with all the bells and whistles. Angel eagerly accepts, but with a bit less concern over Martes himself than the old Angel would have had. They meet, with some flirtation (Martes notices Angel is wearing Calvin Klein aftershave, but it's really Pessi's juniper-berry scent), and Martes provides Angel with Stalkers in a range of sizes. When he comes home from his meeting Palomita, the mail-order bride, is waiting for him. Afraid her abusive husband has come home, she darts into his apartment and sees the troll. From this moment she shares Angel's secret. Pessi, however, has become ill-he's listless and feverish, and Angel realizes something must be done. So he meets his ex-boyfriend, the veterinarian, who diagnoses the undisclosed mystery animal with worms. Angel convinces the vet to take him back to his practice for a tryst and steals a worm medication while his lover is sleeping-which luckily works. When the vet realizes the drugs are missing, he calls Angel and hints broadly that he knows he must have a troll.
As Pessi's health improves, he begins building a pyramid with some blocks that Angel uses for photo shoots. Angel's curiosity over exactly how intelligent his troll might be draws him to continue his research, and he even believes that "devil's stoves," supposed Bronze Age cremation pits built by humans, may be what Pessi is building. The troll's happiness is infectious, and, feeling restless and full of nameless desire, Angel goes out on the prowl and meets a young man who's long idolized him from afar, Ecke. A slightly nerdy, intense young man, particularly alongside the beautiful Angel, he's not the type Angel would ordinarily go for, but he goes home with him. Ecke even agrees to lend him a very rare 19th-century book on the wild animals of Finland, which he wouldn't let out of his sight for anyone else.
When Angel returns home from Ecke's, he finds it increasingly difficult to ignore that Pessi exerts some sort of erotic charge on him. He tries to get rid of Pessi, but Pessi's coat has molted because of the heat of Angel's apartment, and he looks upon Angel as his alpha, or father. In the end he can't bring himself to abandon him. However, he does have a stroke of brilliance: Pessi as the model for the Stalker campaign. A child's size pair, with a tail hole cut in them, works perfectly, though Pessi is instantly furious. The resulting images, of Pessi dancing and leaping with rage like a more acrobatic wild cat fighting its first collar, are exactly the image of the unloosed id Angel is looking for. Martes is delighted-especially since, to protect his secret, Angel tells him the world rights are available and they are only Photoshopped from old nature photographs taken by his deceased brother.
To escape the closeness and disturbing feelings of his home with Pessi, Angel goes home with him, but Pessi smells the other man and growls in a display of agitation until Angel showers. The same evening, Martes, drunk with the triumph of his "Mark of the Beast" Stalker campaign being enthusiastically bought by the jeans company, and following some unacknowledged attraction to Angel, decides he has to return the CD with Angel's photos. Unfortunately, he shoves his way in to use the bathroom when Angel refuses, and Pessi attacks him in a jealous rage. Martes knocks Pessi unconscious.
Spider, the veterinarian ex-boyfriend, comes over, the truth finally out, and treats Pessi for his injuries and catatonia. He also tells Angel that his apartment reeks of pheromones, which could explain his particular irresistibility lately-but which will also betray him to anyone who might suspect him of having the forbidden animal. Pessi's hold is even stronger after the trauma of thinking he might be seriously wounded, and Angel goes to Ecke's immediately. On the way out, he sees that Pessi has painted something resembling cave art in the hallway, using Martes's blood.
The Stalker campaign hits the newsstands and is a huge hit. Except with Pessi, who tears the images to shreds in fury, and Ecke, who recognizes the troll image from a rare Tom of Finland book of erotic drawings featuring the animals. But Martes's revenge is only days away: he's taken another of the troll images and, broadly interpreting the contract Angel signed, used is as the logo of a sports team without Angel's consent. Angel bolts from Ecke's, leaving his keys behind, and makes his way across the city furiously shouting at Martes on his cell phone-who threatens to reveal the truth about the troll to everyone. Ecke, seeing Angel's keys, decides to surprise him. Going to get spare keys from the building super, and being attacked by Palomita's jealous husband, who thinks they are having an affair, Angel misses by minutes the opportunity to save Ecke from Pessi's jealous rage. But it's too late. Ecke is dead.
Angel goes on the run, with Pessi wrapped in a blanket. He takes a cab to the edge of the forest. As he disappears, the police, alerted by a neighbor that there is a dangerous animal in Angel's apartment, arrive to the bloody scene at his apartment. Spider happens to be the consulting veterinarian to deal with the animal and has already told Angel to disappear. The book closes with Angel wanted for homicide, and disappearing so deep into the woods he will never be found, a captive of Pessi's band, headed by an enormous, mature male troll toting a stolen military rifle.