Andy (LUKE WILSON) is a self-described serial monogamist and classified ad writer who's recently been dumped by his girlfriend, Cheryl (KATHLEEN ROBERTSON). She's moved out with their dog, Mogley, in favor of Trevor (GORDON CURRIE), a punk rocker with a penchant for routinely changing his hair color. The fact that they have joint custody of the dog, however, sends the recently split couple and their pooch to Dr. Cavan (MARK McKINNEY), a renowned animal psychologist.
Having dinner at a singles bar, Andy's bad fortune seems to change when he meets Lorna (NATASHA HENSTRIDGE), a.k.a. Ms. Bookworm, the host of a children's TV show. She's recently been dumped as well, and despite her believing Andy's genuine friendliness to be a come on, she returns home with him. Even so, nothing happens as she gets sick from a bit too much drinking that night.
Andy immediately falls for her, but she's embarrassed about the night before and decides not to see him again. This is despite the advice of her friend, Rachel (AMIE CAREY), who tells Lorna that dating gets tougher the older one gets. Nonetheless, Lorna ends up going out with an odd character, Callum (HARLAND WILLIAMS), who picks her up in a video store.
Meanwhile, Andy's good friend and coworker, Jeri (JANEANE GAROFALO), and her steady boyfriend, Jeff (BRUCE McCULLOCH), encourage Andy to return to the dating scene. As such, and after being bought by Keiran (KRISTIN LEHMAN) at a charity auction, Andy starts dating her and she soon moves in with him.
He still longs for Lorna, however, and upon seeing her and her dog, Peanut, in the local dog park, hopes that something might work out between them. As this continues and the dog owners prepare for their pets' big graduation day from obedience school, the romantic lives of all those involved become more complicated than any initially imagined they could be.