One day while walking in a New York skyscraper, Mark Fletcher spots his high school crush, Katie Bowman, boarding an elevator. He tries to get on but the doors close. He quickly races up to the next floor to get one, but the doors close before he makes it. He then races up yet another flight of stairs and finally gets on. She instantly recognizes him. Mark gets on the elevator and they talk about what Katie has done recently. As the elevator reaches another floor Katie gets off. Mark asks if she still lives around here and she says that she lives with two roommates in Brooklyn and she is looking for another one. Mark says that he could be her roommate and she happily agrees.
The next morning, Katie is seen in her room with her on and off again boyfriend Ben. Ben is getting dressed and he and Katie start to argue about whether or not they had make-up sex or break-up sex. Katie tries to make him talk, but instead offers to have sex again. After they are done she tries to get Ben out of the apartment before her roommate Hope finds out, but he blows it by saying goodbye. Hope then tells Katie that she needs to forget about Ben and move on already. Katie agrees and says that she envies Hope and how successful she is.
That same morning, Mark is sleeping on the couch in his and his friend Thom's apartment. Thom tried to wake him up, but Mark tells him to go back to bed. Thom says he can't, because he is too excited. He then pours some cereal into a pot of chicken coup and east some. Thom goes over to the couch to wake him and they briefly wrestle, until Mark realizes that Thom is naked. He then gets up.
In a coffee shop, Hope's boss comes up to her and tells her that she forgot to wear her name tag. Hope becomes angry saying that she didn't have to wear a name tag or fake smile at her last job, to which her boss replies that is probably why she was fired. During this, Mark walks in. Hope takes Mark's order for Green Tea. Thom and Mark begin to discuss Mark's plan once he moves in with Katie and Thom criticizes it. Hope gives Mark a cup, but it is coffee instead of Green Tea. Mark tells her this, but Hope denies he order tea. They get into a little argument, and Hope finally gives Mark some tea, and a "reward" for being an anal customer. They continue their argument and Mark tells her to learn how to be a barista.
Mark goes to a roommate interview where Katie's roommate James interviews him about his humor, reasons for moving in, etc. She realizes that that Mark really wants to move in. Mark tells him that him and Katie are just friends, and James begins to toy around with him about Katie. He finally gives Mark a contract that he needs to sign. Mark signs it and he is now their roommate.
At a bar, Katie and Hope meet up where they talk about their busy days. Mark walks in, and Katie goes and hugs him. Katie goes to introduce them. Mark and Hope see each other and are shocked to see each other again.
Later, Katie helps Mark move into the apartment and they reminisce about High School. Katie decides to ask Mark about advice regarding her relationship with Ben. However, Mark does not realize this and believes that she is talking about him.
At Thom's apartment, Mark continues to move his stuff out, and he tells Thom about his talk with Katie. Thom then reminisces about his and Mark's time together in college and all the other memories they have had. Mark then gets ready to leave and Thom begins to cry.
Mark goes to the apartment. Ben is there trying to leave, and he bumps into the door spilling Mark's drink on his pants. Ben leaves and then Katie talks to Mark and thanks him for his advice with Ben. Katie notices the drink on Mark's pants and tells him to take off his pants quickly before they stain. He is shy at first but eventually does.
Hope and James walk in and Hope notices something wrong with the couch. Katie tells Mark that he and Ben were "naughty" on the couch. Mark takes the blame for this, but when he goes into the living room he realizes what he has gotten himself into. James then walks out saying there is tissue on his bed. He sees Mark in his underwear and freaks out. He goes into the bathroom and brings out a pair of handcuffs. Mark says that he likes a challenge and Hope and James leave disgusted.
Sometime later, Mark, Katie, James, and Hope are having a meal at the table. No one is saying anything. Mark sneezes into his hand. James then jokingly grabs the tissue box and offers him a tissue. Embarrassed, he grabs one.