To take down South Boston's Irish Mafia, the police send in one of their own to infiltrate the underworld, not realizing the syndicate has done likewise. While an undercover cop curries favor with the mob kingpin, a career criminal rises through the police ranks. But both sides soon discover there's a mole among them.

Drea, a 20-something woman living in Brooklyn, struggles to make progress in her own life while being the sole caregiver for her father, who has Alzheimer's.

Kelly's grandpa invites the whole gang to his little hotel "The Hideaway" in Hawaii for summer vacation. They expect a marvelous time at the beach - without Belding. Upon arriving, they quickly learn that not only is Belding there, but they also have to help Kelly's grandpa from losing his hotel. Meanwhile Zack meets (again) the love of his life, Jessie and Slater bet with Lisa that they won't fight, and Screech gets kidnapped by the Pupuku clan.