Billie and Zack discover the gender of their child – thanks to Abby.Billie and Sully have to work on a story, but due to her embarrassing him previously, Sully is reluctant.While Billie receives encouragement and advice from her alcoholic 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.In early 2010 CBS moved Accidentally on Purpose to Wednesday nights from Mondays, to accommodate the midseason return of Rules of Engagement and the cancellation of Gary Unmarried.However, things take a turn when the objects of the boys' affection reveal their true intentions.Meanwhile, James tries to hook up with teenagers by pretending to be one himself.
While snooping through Zack's belongings, Billie discovers a note addressed to her that he never finished writing.
Zack intends to meet some other fathers as most of his other friends are not at this stage of life yet, but instead makes friends with a lesbian couple. While Zack shines in his class, Abby outshines Billie at all the tasks.
The lesbian couple guides Davis to another lesbian couple to help him get laid, which yields unexpected results. Worried that she will ruin Christmas, as she has in previous years, Billie tries to hide from her parents the fact that Zack is living with her.
Meanwhile, Abby's husband, Nick, tries to impress their father.
Zack tries to get along with Nick, but Nick is too strange for him.