The following version of this book was used to create the guide: Armas, Elena. The American Roommate Experiment. Simon & Schuster, 2022.

When the ceiling of Rosie Graham’s apartment caves in, she takes advantage of her best friend’s spare key. Lina is on her honeymoon with her husband, Aaron, and is not replying to Rosie’s frenzied calls. As Rosie is settling into Lina’s studio apartment, someone else tries to get in. Rosie assumes it is a robber and calls the police. It turns out to be Lina’s cousin, Lucas Martín, to whom Lina promised the apartment for a few weeks.

Rosie feels starstruck meeting Lucas: her Instagram crush. She was disappointed when Lucas did not attend Lina and Aaron’s wedding. Lucas is a surfer recovering from a recent injury. Lucas insists Rosie remain in the apartment while he goes to a hotel. Rosie finds him the next morning sleeping inside a diner. She was on her way to Dad’s place to meet her brother, Olly, who she is worried about. When Lucas wakes, they chat. He says he cannot go to a hotel because he is waiting on a credit card replacement. Rosie tells him how she recently quit her engineering job to start a career as a romance writer.

After a brief visit to Dad’s, Rosie returns home to find Lucas cooking. Rosie feels awkward and shy. He proposes they live together for the duration of his stay. He will go out sightseeing while Rosie writes during the day Rosie consents. After dinner, they watch TV together. Rosie picks a vampire show for them.

The next day, Rosie’s writing is interrupted by a strange noise outside. Adele, Lina’s elderly neighbor who suffers from Alzheimer’s, is attempting to move furniture by herself. Lucas comes to the rescue. He does not contradict Adele when she calls him Mateo: her dead husband's name.

Lucas asks Rosie for a hug afterwards. Adele reminds him of his grandfather, who also had Alzheimer’s. Lucas cooks them dinner. They watch their show.

The next day, Lucas bumps into Rosie outside Lina’s apartment after sightseeing. He accompanies her to see her landlord about the damage to her apartment. The contractor, Aiden, is handsome and kind. Rosie breaks down that night about the chaotic state of her life. Lucas reassures her that it is okay to lose control sometimes.

The following day, Rosie is too distracted thinking about Lucas to write. When Lina finally calls, Rosie conceals that she is staying with Lucas at Lina’s place. Lucas interrupts the call to present Rosie with cronuts: the food she was craving the night before. He asks her about her exes. She tells him about some of the toxic men she has dated. Lucas suggests he be Rosie’s experimental boyfriend. He hopes it might help her get over her writer’s block. They agree to go on four dates. Lucas promises not to fall in love with Rosie.

One week later, Lucas takes Rosie on their first date to a record store. He has her pick out their soundtrack. Rosie selects ABBA’s Dancing Queen, which they dance to together in Lina’s apartment. They are interrupted by the sight of a rat. Lucas catches it in a box. Just then, a woman knocks on the door. It is Alexia, Adele’s daughter. Alexia thanks Lucas for checking in on Adele every day.

Olly does not show up to Dad’s but waits for Rosie at the train station afterwards. He has a black eye and explains that he is a stripper. His boss arrives and takes him away in a car. Rosie is distressed when she gets home, so Lucas hugs her.

The next evening is Rosie and Lucas’ second date. Lucas takes her to Alessandro’s: the pizza place across the street. It is empty except for a path of candles lighting the way to the kitchen. Lucas explains that they are cooking pizzas for their date since he knows how much Rosie likes watching him cook. They flirt and nearly kiss before they notice their pizzas are burning. Back at Lina’s, Rosie orders them takeout. Lucas is embarrassed by his failed date.

Lucas emerges from the shower the next morning to find a very angry Lina accusing him of sleeping with her best friend. Rosie does her best to explain the situation. The women leave for their favorite café to chat. Lina warns Rosie that Lucas is an expert charmer. Meanwhile, Lucas reassures Aaron he would never hurt Rosie. That evening, Lucas cooks dinner for Rosie, Alexia, and Adele. Alexia—a top chef—insists that Lucas is an incredible cook.

Aaron and Lina invite Lucas and Rosie to a Masquerade Ball. Lucas and Rosie go as their favorite couple from their vampire show. Lucas feels protective of Rosie at the ball and goes with her to the bathroom when someone spills a drink on her. Lucas closes his eyes while Rosie changes. She orders him to open them. He starts touching her but is interrupted by Lina’s arrival. Rosie exits the stall alone to save face.

A week later, Rosie is disappointed that her bathroom rendezvous with Lucas changed nothing between them. While writing in her café, she sees Aiden. Lucas shows up just as Aiden is asking Rosie out on a date. Rosie declines his offer.

As they are walking back to Lina’s, Lucas is faced with his sister, Charo, and his dog, Taco. Rosie secretly arranged for them to surprise Lucas, moving him deeply.

Lucas takes Rosie to Alexia’s restaurant for their next date. After a delicious dinner, they head to the rooftop greenhouse. Lucas serves Rosie the exact cake from Aaron and Lina’s wedding. He explains that he wanted to recreate the atmosphere since Rosie once said she was sorry they did not meet then. When it starts raining, Lucas offers Rosie his jacket. She cries. She tells him she wants him to want her in the same way. They kiss. As they discuss having sex, Olly calls. He needs them to pick him up from the nightclub he works there.

Lucas and Rosie rush to the nightclub. Rosie makes Lucas promise not to get into a fight. Olly’s boss, Jimmy, does not want to let Olly leave. Jimmy trips Lucas, making him fall on his injured knee. Rosie threatens to beat Jimmy up with her purse. Lucas tells her to let it go. The trio escapes.

That night, Olly explains that he got into stripping to be near a girl but now wants no part of that life. He sleeps on the couch while Lucas and Rosie take the bed. As they lie together, Lucas relives the gruesome details of his surfing accident. Rosie reassures him that he is not broken, only different.

After dropping Olly off at Dad’s, Rosie returns home to find Lucas masturbating in the shower. She masturbates with him. Later that day, Lucas and Lina help Rosie move back into her now-repaired apartment. That night, Rosie texts Lucas to tell him she misses him. He arrives at her apartment shortly after. They have sex.

The next morning, Lucas leaves for his flight back to Spain without saying goodbye. Rosie shows up at the airport and offers to leave with him. Lucas says that she should not uproot her life just for him, then leaves.

Heartbroken, Rosie takes comfort in spending time with her family and Lina. She finishes her book, in which Lina recognizes traces of Lucas. Lina calls Lucas to call him an idiot and explains that Rosie is in love with him.

Months later, Lucas shows up at a New Year’s Eve Party to profess his love to Rosie. He explains that he wanted to take time to work on himself in order to be his best possible self for her. He requests their song, Dancing Queen, and asks her to be his girlfriend. Rosie says yes. Lucas and Rosie date long-distance for a year while Lucas waits for his acceptance into a New York City culinary school. He surprises Rosie by arriving a day earlier than he said he would.