Kassie Larson, a single woman in her early forties, decides to take her life into her own hands by opting for artificial insemination to have a baby. Seeking the perfect sperm donor, she settles on Roland, a charming and seemingly ideal candidate. Kassie's best friend, Wally Mars, though supportive on the surface, secretly harbors deep feelings for her. Wally, who is neurotic and somewhat pessimistic, drunkenly meddles with the insemination process during Kassie's insemination party, inadvertently replacing Roland's sperm with his own. Unaware of this switch, Kassie goes through with the procedure and eventually moves away, leaving Wally to deal with his complex feelings alone.

Years pass, and Kassie returns to New York with her young son, Sebastian, whose peculiarities and neuroses strikingly resemble Wally's. As Kassie and Wally reconnect, Wally starts to notice the similarities between himself and Sebastian, leading to memories of the inebriated mishap. Wally grapples with the decision of whether to reveal the truth about Sebastian's paternity to Kassie and how it might affect their friendship. Meanwhile, Kassie begins dating Roland, further complicating Wally's emotional turmoil and his burgeoning relationship with Sebastian.