Harley tries her hand at running a day camp for neighbourhood kids in the Diaz backyard in an attempt to raise some money, but finds herself distracted by Ellie's annoying cousin.
Charlie and Jake investigate Nikolaj's family when a visitor from Latvia shows up unexpectedly. Meanwhile, Holt and Amy request famed forensic scientist Dr Yee's help to solve a case for Rosa.
Jake runs into Jess Day, who is visiting from Los Angeles, and enlists her help on a case; Holt looks for ways to improve morale; Amy investigates Rosa's long breaks.
When Jake helps Charles track down his son's favourite toy for Christmas, the pair end up facing off against a Latvian crime ring. At the precinct, the team gets ready for the annual carolling contest against its sworn enemy, the MTA.
When Pimento and Rosa decide to tie the knot, the squad prepare a wedding under Amy's direction. Jake and Gina help Pimento find his grandmother's ruby earrings.