Amy (Dever) and Molly (Feldstein) are high school seniors who have been best friends since childhood. As graduation draws nearer, they realize that their years of working hard and steering clear of parties may have been unnecessary, as even the party kids have been accepted to prestigious colleges. The girls decide to throw their bookish, pretentious personas out the window for one big end-of-year bash and soon find themselves at a classmate's yacht party completely unsupervised. Molly and Amy agree to go wherever it takes them, and after being picked up by their principal (Sudeikis) via a ride-sharing program, the night takes some weird turns. The girls stick together through much more than they bargained for.
After alien enthusiast and FBI reject Molly Wright (Tessa Thompson) sneaks into the MiB headquarters in New York City, she makes a noteworthy impression on Agent O (Emma Thompson) and is granted provisional agent status as "Agent M" in London, England, working under Agent H (Hemsworth) and his mentor High T (Neeson). After a late-night meeting with Vungus the Ugly (Novak), Agent H and Agent M are confronted by a pair of alien twins. When Vungus is killed in the attack, the agents must unravel a sinister plot and uncover a possible traitor inside the MiB.
After Maggie Thompson (Silvers) moves to Ohio with her mother, Erica (Lewis), she befriends a new group of teenagers, who encourage her to drink with them. The group enlists a local veterinary technician, Sue Ann Ellington (Spencer), to help them procure the alcohol. Although Sue Ann anonymously reports the teenagers, she still invites them over to drink in her basement, and her home becomes the location of a house party, making her popular with the teens. After a while, Sue Ann's generosity begins to scare the high schoolers, who suspect she is not who she seems.