James Mitchum finds himself living in the confined space of an underground bunker alongside his claimed parents, Ted and April. As Ted regales him with cautionary stories about the dangers lurking outside, James can’t help but feel a sense of wonder as he glimpses Ted daringly stepping into the unknown, clad in a gas mask. His escape from the mundane reality is found within the colorful world of Brigsby Bear, a children's show featuring the titular bear who assists characters in navigating perilous circumstances. This show, strictly allowed by Ted, becomes James' sole source of joy and comfort.
One fateful evening, driven by curiosity and longing for freedom, James decides to sneak out of the bunker. But what awaits him is the blaring of police sirens, signaling impending trouble. The authorities descend upon the bunker, resulting in James being pulled away from Ted and April, who are soon taken into custody.
The next phase of James' life begins in the police station, where he meets Detective Vogel, a determined investigator who disrupts his understanding of family by revealing that Ted and April are not his biological parents. She introduces him to his real family—Greg Kinnear, his biological father, and Louise, along with their teenage daughter, Aubrey. The harsh truth of James' isolation since childhood begins to sink in. As he struggles to acclimate to this new reality, he finds solace in Emily, a compassionate psychologist, who helps him navigate his feelings about Brigsby Bear. It's revealed that Ted, once an artist and designer, created the show with his wife in 1987 before they mysteriously vanished, leaving only their beloved creation behind. Unbeknownst to James, the authorities had been hunting for them based on tips from concerned citizens.
With a strong desire to carry on the legacy of Brigsby Bear, James resolves to make a film inspired by the character. Aubrey steps in as his ally, connecting him with Spencer, a budding filmmaker who shares his enthusiasm for storytelling. Detective Vogel, moved by James' plight due to her own aspirations in acting, generously provides him with props from the show, fueling his creative spirit. As they team up, excitement builds around the film project, especially when episodes of Brigsby Bear find a new platform on YouTube, garnering a fresh fanbase. However, Greg and Louise, while supportive, worry that James' cinematic journey might hinder his chances for a regular life.
As they delve deeper into the filmmaking process, an unexpected explosion occurs during a scene shoot in the woods due to a prop concocted by James. The fun of filming suddenly turns into chaos as police officers converge on them, transforming excitement into dismay. In a moment of bravery, James takes full accountability for the incident, which leads to his temporary release but results in the confiscation of vital Brigsby Bear props once again. Under the cloak of night, he makes a daring decision to steal his parents' car, embarking on a quest to reclaim the cherished items. A visit to his former underground home, now deserted and sealed off, becomes a poignant reminder of his past. Afterwards, he visits a diner where he fortuitously runs into Whitney, an actress who starred in the Brigsby Bear series. During their reunion, she admits her ignorance about the true nature of her role, having believed she was acting for a Canadian public access show. With urgency, James asks Whitney to reprise her character in his film as the police close in.
Meanwhile, Aubrey's parents witness the creativity James has uncovered through his film screenings, providing a glimpse into how these experiences have drawn him closer to his friends. Just when hope seems lost as James finds himself in a mental institution, a chance to escape presents itself one fateful night. Upon regaining his freedom, he discovers that his family, along with Spencer and Vogel, have been diligently constructing a Brigsby Bear set in their garage, inspired by the joy they observed during filming.
With Vogel and Whitney stepping into significant roles in his production, James pours all his energy into completing the film. He pays a visit to Ted in prison to offer an apology for his past actions related to his abduction. Their conversation reveals James' struggle to perfect the tone for Brigsby's voiceovers, prompting Ted, filled with regret, to assist by recording the necessary lines.
As the premiere day arrives, an anxious James grapples with doubts about how his film will be received. Choosing to remain in the shadows while the audience watches, he finally gathers the courage to step into the theater just as the credits roll, greeted by an overwhelming standing ovation. The warmth of his family's embrace fills him with pride as they celebrate his accomplishment together, and in an extraordinary moment, he catches a glimpse of a real-life Brigsby Bear on stage, its presence both magical and fleeting.
