16-year-old Monroe “Mo” Harris is escorted out of class by a police officer.
When Mo was younger, his older sister Kate dated her classmate Zeke Presanti. Even after Kate broke up with him, Zeke and Mo remained close friends, despite their age difference. Mo’s parents, Reuben and Sherri, allow Mo to continue hanging out with Zeke despite their concern about Zeke’s influence on their son. Zeke, now 23, works a dead-end job and spends most of his time smoking marijuana and drinking.
Mo spends most of his time with Zeke, his friends, and his girlfriend Holly rather than anyone his own age. Zeke helps Mo acquire drinks for a senior party after being invited by fellow sophomore Stacey, and gives Mo some weed to sell to the other kids. This earns Mo credibility among the seniors at the party. Mo also sees his classmate Sophie there, and they strike up a conversation and exchange numbers.
Mo later takes Sophie out on a dinner date before taking her back to Zeke’s place to hang out. Before Sophie leaves, she and Mo kiss. This earns him the nickname “Tongue Daddy” from his friends, which Zeke tattoos on his chest. Mo comes home accidentally stoned the next night before a family dinner, and, after his father finds out about this and his tattoo, Mo is grounded and forbidden from hanging out with Zeke.
Mo continues to supply drugs and alcohol at the parties. Though he has doubts about his actions, Zeke encourages him to and continues to supply him, even quitting his job due to the amount of money Mo makes off of selling.
Mo begins to ignore Sophie, per Zeke’s advice on how to get girls, which makes her mad. After realizing his mistake, he tries to explain himself but she refuses to listen and walks out on him. Mo gets a call from Holly and he goes over to her place. Holly tells him that she and Zeke have broken up because he was cheating and she takes Mo’s virginity. After the break-up, Zeke spends his night getting drunk and high, and sings in a karaoke bar.
Stacey drives into a ditch with a few seniors who are smoking and drinking and abandons his car. The next day, a police officer comes to school and interrogates him, telling him he won’t get in trouble if he reveals who supplied the drugs they found in the car.
When the cops show up at a party looking for Mo, Zeke manages to get him out and bring Mo back to his place. Mo cries about how he has become a degenerate like Zeke, even though he never wanted to be. He also admits to Zeke that he slept with Holly, but Zeke forgives him and the two reconcile. Zeke offers to take the fall, but Mo declines his offer, saying Zeke has done enough and he needs to handle this on his own.
The following day, Mo is escorted out of class, gets expelled from school, and is sentenced to community service. Reuben drives to Zeke’s house and orders him to stay away from Mo.
Three months later, Mo stops at a fast-food restaurant one day and is surprised to see Zeke working at the drive-thru window. Mo sits with Zeke inside to catch up. Zeke asks Mo if he wants to hang out the next day and Mo says that maybe they can. Mo drives away, leaving Zeke behind.