Accidentally on Purpose follows Billie (Jenna Elfman), a San Francisco movie critic in her 30s. Billie meets Zack (Jon Foster), an aspiring chef in his 20s, at a local bar and they have a one-night stand. She soon finds out that she is pregnant, and decides to keep the baby. She must simultaneously deal with her boss and ex-boyfriend, James (Grant Show), who finds himself jealous of Zack. Before the story began, she had hoped that James would ask her to marry him, but he told her that he was not ready because he was married once before.
Thanks to this unexpected arrival, Billie and Zack have agreed to live together platonically, since the only place that Zack has called home was his van. While Billie receives encouragement and advice from her alcoholic girl best friend Olivia (Ashley Jensen) and her conventional, younger, married sister Abby (Lennon Parham), she also has to deal with Zack's freeloading friend Davis (Nicolas Wright), among others, when they start turning her place into a frat house, leaving Billie to question whether she is living with a boyfriend, a roommate, or a second child to raise