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
Billie Chase is a successful 30-something movie critic living in San Francisco who has just broken up with her boyfriend, James, a very wealthy media mogul who is also her boss. She ends up rebounding with a much younger man, Zack, who at 22 years of age is just starting out on his career as a chef. Billie and Zack’s fling takes a serious turn when Billie discovers she is pregnant. Not knowing what the future has in store for her, Billie decides to keep the baby and raise it by herself if necessary. However, Zack decides to step up and help Billie with the baby and after Zack loses his rented room, Billie invites him to move in with her. None of this sits too well with James, who tries to win Billie back only to discover that she’s now very involved with Zack.
As Billie looks through Zack's things, she finds an unfinished note intended for her. She attempts to discover what Zack is keeping from her.
Billie's pregnancy causes her breasts to increase in size which causes other guys to flirt with her. Meanwhile, Zach refuses to have sex with Billie after she trys to have sex with him.
Billie and Zack discuss their feelings on dating and agree to see other people, however, Billie's pregnancy gets in the way of any potential love connections.
Billie gives Zack and Davis dating advice. Their plans don't go smoothly as the women they are pursuing bare their real interest.
Zack attempts to surprise Billie by installing a low-flow toilet, but instead they have their first argument as a couple.
Billie requests Abby and Nick to be Godparents, but unintentionally, it causes them to break up. However, Billie has to stop Abby before she does something she will regret when a night with Davis on the town goes too far.
When Billie begins seeing another man, Zack starts to show his jealous tendencies.
After James hires a new film critic to the paper, Billie feels that she has been threatened, and decides to show James how useful she really is.
Billie and Zack begin taking instructional parenting classes, but separately due to their busy schedules. However, Billie decides to go with Olivia and Abby, but when Abby starts to outshine Billie at all the tasks, the course turns into a competition.
Billie tries to cover up the fact that Zack is living with her from her parents, while also worrying that she'll ruin Christmas, just like she has in the previous years. Meanwhile, Nick (Abby's husband) attempts to impress their father.
As they prepare for a surprise shower, Nick and Abby set up their significant others to keep Billie and Zack occupied. The plan turns into a disaster when the arranged couples don't get along.
Billie and Sully work on a story together which leads to a surprising discovery about the paper. Zack begins a new work adventure with Davis as his partner.
Billie tries to manipulate Zack so he does more chores around the apartement, but her plan backfires when Zack ends up decorating the nursery instead. Menawhile, Davis gets a job at a gay bar to earn extra cash.
Billie invites herself to Zack's high school reunion, but with regrets when she becomes unsure if she can't compete with Zack's hot ex-girlfriend in her pregnant state. Meanwhile, Abby makes a miracle youth juice that she tries to sell at a party that she and Olivia host.
Zack is upset when Billie hires a baby nurse without consulting him. Meanwhile, Davis and Ryan are faced with a difficult choice when they learn surprising news about the nurse.
In the first part of a two-parter, Billie acts as if she gets into labor in order to save Zack from a traffic ticket of speeding. Meanwhile, Olivia and Davis spend the day shopping for a baby gift.
After faking labor pains to get Zack out of a speeding ticket, Billie goes into labor - for real. Unfortunately, things take a turn when Zack gets busted by the officer they had initially lied to and is hauled off to the police station.