The film begins with Evan, a young man, discussing his mother's past funeral with his only friend at a restaurant. While they are discussing their lives he overhears a verbally explicit argument between a man and the woman he is with. Evan excuses himself from his friend and goes to work back at the restaurant only to come face to face with the man who was overheard arguing. The man attacks him, obviously being drunk, and Evan beats him savagely to the ground resulting in him losing his job. Despite pleading not to be released, his pleads fall upon deaf ears though a friendly co-worker says he will try to convince the boss to rethink his plans. On his way home, Evan finds that he is being followed by friends of the man he beat down who tell him that they know where he lives. Fearing that he will be sued, or attacked further along the way, and with no job or family left here, Evan considers leaving the city. His friend encourages him to spend the night with a girl and despite initially refusing, Evan does sleep with a nameless girl that night.The next morning, when the girl discovers that Evan feels no romantic feelings toward her after their activities, she leaves coldly. After an elongated state of drunkenness, Evan hears some knocking on his door and looks out the window seeing the cops have arrived and believe he is not home since he has not answered the door. Evan then seizes his chance to escape and books a blind flight to Italy. Upon arriving at Italy, Evan has a drink with some friendly drinkers and befriends them, smoking drugs as well. Life is peaceful for Evan over the next couple of days as he takes in the beautiful scenery as he takes a road trip with his new friends. Upon arriving at a nearby town, Evan sees a bewitchingly beautiful young woman named Louise who notices him as well, and an attachment between them immediately blossoms.While drinking with his friends, Evan sees Louise again and introduces himself to her. She immediately comes onto him and asks him to drink with her. Her sudden interest in him both startles and delights Evan, but he keeps his wits about him and invites her to go out with him tomorrow night. His friends decide to leave and ask Evan if he wants to go with them but he refuses saying that he will stay here (hoping to meet Louise again). Evan then takes a job at a local farm and lives in a nearby town. He quickly befriends his farmer employer named Angelo, and meets with Louise again, asking her to go out with him once more as she refused initially. The two visit an art museum and Evan learns that Louise is an art student. The two go out for dinner where they learn more about each other. Evan also learns that Louise has been all over the world and has been with some other men, none of which she could cope with. He also learns that she has difficulty believing in God. He playfully writes a love letter to her, which she asked him to write for her cheekily, but receives it very graciously.The two continue to speak and drink and eventually both become tipsy. They kiss and then go back to his apartment where they have sex. He takes out a condom but she casts it out of his hand before they proceed. She awakens the next morning as a hideous creature and flees his home, eating a cat on the way out. Louise begins transforming hideously but takes some medication to postpone the transformation. Evan and Louise later meet again and continue their relationship. Evan learns that she can speak numerous languages and they visit her bedroom. She encourages him to quit smoking before going upstairs for a shower. She begins transforming painfully into an abominable monster but counteracts the change via her medication. The two promise to meet again at the beach later and while using her bathroom, Evan sees a syringe on the floor.While working with Angelo the next day, he realizes a strange growth is happening on the tree's leaves and Angelo points out that "Spring" is the cause of change here. Evan meets with Louise and confronts her about the syringe to which she explains it is to inject herself when a condition she has takes over. She asks him about his family to which he becomes uncomfortable and refuses to talk to her about it. An argument erupts and then later Evan explains that his mother had died of cancer last week having been diagnosed soon after his father died. He felt like nothing as he could not help his family and feels he has made nothing of himself "like a sociopath, wondering what I'm going to do for work while his family died." Louise points out that he has the same backstory of Batman, to which he finds little amusement. He inquires about her contacts to which she explains she wears them to look more beautiful. He questions her truth, but she explains she has never lied to him at all. Later, Louise finds that her mutation is growing stronger and stronger despite her medications. She enters into a cave and conducts a bizarre ritual to counteract the process but this also seems to not work.Evan visits her room that night to find her in a sick, exhausted state and tries cheering her up. Later they have dinner, but her skin condition worsens and she leaves abruptly. A man follows after her upon seeing her in pain and tries to take advantage of her but she transforms into a beast and kills him. Angelo watches this from a distance. The next day, Angelo pulls Evan aside and has a heart-to-heart talk with him, telling him that he is working illegally with him as an illegal immigrant and that the police will soon be after him. With nowhere else to turn, Evan goes to Louise's house but finds her in worry. She fears for his safety from her monstrous alter-ego and tells him to leave and forget about her. He reluctantly complies, punching a brick wall on the way out, injuring his hand. He then later returns, knowing that Louise could not possibly mean what she has said and to his horror, finds her in a corrupt state of mutation into a tentacled reptilian creature.Frantically, he gives her an injection and asks if she is a werewolf or vampire, and asks her to explain her condition to him. He leaves in a huff, but Louise follows after him, explaining that she has been living for thousands of years, and that she did not use a condom while having sex with him so that she could purposely get pregnant. She reveals that if she falls in love with someone, her body will produce a chemical that will fight her transformation process and she will lose her immortality. He asks if she would give up her immortality for him and she says no. Feeling used and betrayed, Evan leaves to clear his head. He calls up his friend for some advice but receives no reception. Taking it upon himself to make things right, he reunites with Louise to convince her that she loves him.He asks her to spend her last day with him on a road-trip. She refuses saying that she cannot change the transformation. But when the immigration police arrive, she changes her mind and helps him escape. They drive off to Pompeii and talk all night along the way. The next morning, she tells him of her family history and her voice returns to its natural accent. Evan tries to convince her to choose him over her immortality and they go into a church where she begins to transform with strange but comedic effects. Louise's maturation into the abomination becomes more rapid and the time comes for her to choose either her immortality or Evan, as evident when she inadvertently attacks him. Down to her last syringe, they journey outside at the foot of the volcano. Evan confesses that he knows that he loves her and begs for her to realize her feelings for him, realizing how cold she truly is.Louise grows increasingly fearful of the creature she may become, knowing that she may yet kill Evan. He refuses to leave her, knowing that being mortal is a good thing and convincing her to choose to become human as well. Louise tells him that a huge hideous monster will arise if she goes under and advises him to run for his life if worse comes to worse. He refuses to leave her however, and she reveals that she does in fact have feelings for him and begins crying. The two remain together as she slowly begins changing, and he holds her close. The volcano erupts in the distance and a grotesque sound is heard. Evan looks at Louise and realizes that she has not transformed, and is in a calm, resolute state. It is implied that she chose to become human and remain with him.