*CUT TO THE CHASE*
NOTE: This spoiler was submitted by Jeremy
The film opens with a clip of the TV show "Brigsby Bear Adventures". The titular character is trapped with his friends Arielle and Nina Smiles while the main villain, Sunsnatcher, plans to destroy them. Brigsby uses a magic crystal to defeat Sunsnatcher and free his friends. This is being watched by James Pope (Kyle Mooney), who has his whole room decorated top-to-bottom with Brigsby merchandise, including every tape of every episode.
James lives with a couple named Ted and April Mitchum (Mark Hamill and Jane Addams), who say they are his parents. He is kept in a shelter away from the outside world. After dinner, James goes to the dome where he and Ted look out at a mechanical fox and fake stars that Ted had set up.
At night, James goes up to the roof, only to see police cars approaching. Officers storm the place and arrest Ted and April before escorting James out of there.
James is brought to meet with Detective Vogel (Greg Kinnear). He tells James that Ted and April kidnapped him when he was little and they have pretended to be his parents. James doesn't seem to understand what Vogel is trying to tell him. Vogel then takes James out to meet his real parents, Greg and Louise Pope (Matt Walsh and Michaela Watkins) and their daughter Aubrey (Ryan Simpkins). Greg and Louise are overwhelmed and they go to embrace their boy. On their way out of the police station, the family is mobbed by reporters wanting to speak to James.
James is brought home where Greg and Louise try to help him adjust to the modern world by showing him other TV shows and movies. Greg takes James to see a comedy film and he enjoys it. Aubrey, however, is weirded out by James's nuances.
James and his parents meet with a therapist named Dr. Emily Larson (Claire Danes). At one point, James mentions wanting to know when the new "Brigsby Bear" tape will arrive. Dr. Larson tells James that Brigsby wasn't real. James disagrees because he has spoken to others that have watched the show online, but Dr. Larson says that those forums were most likely created by Ted and April. James then asks about movies and how others make movies, and he then hatches an idea.
James gets a bunch of books on how to make a movie as he comes up with storyboards for a "Brigsby Bear" movie, hoping to give the series a proper conclusion.
Aubrey reluctantly takes James with her to a party. They pick up Aubrey's friend Meredith (Alexa Demie) and head over. James meets her other friend Spencer (Jorge Lendeborg, Jr.). James notices his "Star Trek" shirt and starts telling him about "Brigsby Bear". Others gather around to hear about the series and they seem fascinated. James is given beer and a drug from Meredith. She takes him into the bedroom where they start to hook up, but James becomes nervous as he starts tripping out and he leaves. He leaves the party and hallucinates that Sunsnatcher is up in the sky taunting him.
James goes to visit Vogel over the confiscated props from the Brigsby series. Although it is technically evidence and police property, James appeals to Vogel's love for acting and convinces him to bring him a number of props from the show, including the animatronic Brigsby head.
Spencer joins James's project and brings his camera along while also showing James how to create effects. Spencer even uploads episodes of "Brigsby Bear" to YouTube, leading to others discovering the series and becoming fans. However, the whole thing makes James's parents uncomfortable. One fan approaches James at the store and asks for a picture while also mentioning him being kidnapped.
James goes with Aubrey, Spencer, Meredith, and another friend named Logan (Chance Crimin) to a field where they continue shooting, with James playing the part of Brigsby. Aubrey apologizes to James for how she acted toward him previously. The group later sits around a fire.
James and Spencer film a scene involving an explosive. To Spencer's surprise, James used a real homemade bomb that goes off and creates a huge explosion. Cops arrive promptly and arrest them.
James is interrogated by Deputy Bander (Beck Bennett). James takes the blame for the explosion and for all the props missing from police storage. Bander admits to having seen some of the Brigsby clips online. He ends up letting James go.
Undeterred, James tries to continue working on his project. He seeks out the actress that played the Smiles Sisters. She is a waitress named Whitney (Kate Lyn Sheil). James finds her at a diner where she treats him to some food. Whitney feels remorseful over what happened to James and had no idea what she was involved with. Moments later, the police show up to bring James home.
James argues with Greg over how working on the movie is helping him have a normal life. During a session with Dr. Larson, she recommends that James be placed in a mental institution. The Popes decide it's for the best and they send James away.
During James's stay at the institution, he befriends another patient named Eric (Andy Samberg).
Aubrey shows her parents what she's been working on with James. Greg and Louise are touched to see that this has made James happy, interacting with other people and doing something he loves.
James breaks out of the institution by throwing a TV through a window and climbing out. He runs back home and finds his parents with Aubrey, Spencer, and Vogel in the garage continuing the Brigsby movie on their own. Everyone is set to help James finish his movie.
James visits Ted in prison. He and April have pled guilty to their crimes. However, James appears forgiving to him. He asks Ted for the last thing the movie needs - Brigsby's voice. James takes out a recording device and the script, and Ted starts to read the lines.
A week later, the movie is complete and is set to have its world premiere at a local theater. The event is sold out. James gets nervous that people won't like the movie, and he hides in the bathroom. Spencer tells him it doesn't matter what people think of it.
The movie features Brigsby on a quest to defeat Sunsnatcher once and for all. Vogel plays a wizard while Whitney reprises her role as the Smiles sisters. The audience laughs and reacts accordingly. In the end, Brigsby destroys Sunsnatcher. James walks into the theater as everyone gives him a standing ovation. As Aubrey and her friends go to hug James, he sees Brigsby himself standing on the stage, waving at James before disappearing.