In the Series finale, an old classmate of Cooper and Neal's is interested in buying into Barry's Grandma's Hangover Cure, but Neal refuses to work with the guy due to an incident from their past. Meanwhile, a burst apartment pipe sends Kelly to stay at Josh and Leslie's, where she becomes fixated on finding out what is behind a secret door in their home.
A potential buyer for Barry's Grandma's Hangover Cure turns out to be someone who has had past issues with Neal, while Cooper and Kelly pretend to enjoy dating others in an attempt to make each other jealous, and Neal and Barry begin seeing the same polyamorous woman.
Cooper and Josh's parents visit, putting the guys on their best behavior, until Dad reveals a secret that leads to a crazy night out and a run-in with rapper Juicy J (as himself). Meanwhile, Barry helps Neal deal with an odd addiction.
Barry and Neil become frustrated when Cooper bags all the credit for the success of Barry's Grandmother's Hangover Cure, but a rival CEO attempts to usurp them and become the hangover-cure queen, so the friends must rally to stop her.
Cooper's ex-girlfriend turns up and causes him embarrassing public humiliation, and when Neal tries to help, things go from bad to worse. Meanwhile, Kelly meets a new guy who's as competitive as she; and Josh struggles to keep Leslie out of group hangouts.
Cooper feels entirely disconnected when he loses his phone, and the guys pull out all stops in an effort to help him find it. Meanwhile, a Leslie-and-Kelly karaoke duet in a bar triggers an all-out brawl.
Kelly's invited to her former boyfriend's wedding in Mexico and asks Cooper to be her date, but their south-of-the-border escapade proves a logistical nightmare and he enlists the help of Neal, Barry, Josh and Leslie to get them to the church on time.
With sales of their hangover cure sluggish, the guys seek to score some much-needed cash by taking part in a drug trial, but their plans are scuttled after an unexpected run-in with Paula Abdul (as herself). Meanwhile, Kelly finds herself in an uncomfortable position after hanging out with Josh and Leslie.