John (Reilly) has been divorced for seven years when his ex (Keener) brings him to a party. There, he meets Molly (Tomei), who wins him over. But John finds an obstacle in the form of Molly's son, Cyrus (Hill), who's determined to get John out of the picture.
The film provides examples of the following tropes:
- Amicable Exes: John and Jamie are relatively on good terms for much of the movie.
- A Date with Rosie Palms: At the start of the film, John's ex-wife Jamie walks in on him masturbating to Internet porn and decides to help him find a date.
- Heel Realization: Cyrus finally goes through this near the end of the film when he realizes that by driving John away, he may have his mother to himself again, but now she's not happy.
- Interrupted Intimacy: Happens twice — Molly closes the door of her bedroom (she normally leaves it open) and she and John begin making out before being startled by the sound of Cyrus screaming, having had a "night terror." Later, after Cyrus has moved out, they come home and begin having sex in the living room before Cyrus suddenly turns on a light and announces he wants to move back in.
- Love Triangle: A variation in that John finds himself competing for Molly's attentions with her son Cyrus.
- Manchild: Lampshaded by Molly, who says that 21-year-old Cyrus is very mature in a lot of ways, but in other ways, he has not come into himself yet.
- Meet Cute: Molly approaches John while he's taking a whiz in the bushes during a party, and they joke around before returning to the party and dancing to The Human League's "Don't You Want Me?"
- My Mom Is Off Limits!: The premise of the movie in a nutshell. It's even on the poster.Cyrus: Seriously, don't fuck my mom!
- Never Trust a Trailer: The trailer makes it look more comedic than it actually is. Granted, the movie isn't completely devoid of humor, but it's more poignant than the trailer made it look.
- Single Parents Are Undesirable: Molly has a lot of trouble dating because her immature 21-year-old son still lives with her, and his considerable issues scare off many of her potential paramours.