As Joe Wentworth stands at the grave of his beloved friend, Simon Birch, he begins to recount their heartfelt journey from childhood. A seemingly trivial detail about Simon's height masks the significant influence he had on everyone around him, including his family, who often dismiss him with cruel disregard. Despite being the smallest child in the history of Gravestown, Simon finds a passion for baseball, which serves as both his greatest joy and immense source of pain, as he is frequently left to watch from the sidelines during Little League games.
The arrival of drama teacher Ben Goodrich at Joe's mother Rebecca's dinner table signals the start of a pivotal transformation in their lives. Rebecca's hesitance to disclose Joe's father's name has marred his reputation as the town's illegitimate child, intensifying Joe's yearning for the truth. Meanwhile, Simon feels an instant connection to Ben’s warm and understanding demeanor, while Joe remains guarded, consumed by his quest for his biological father.
As the trio heads to church, Simon's passionate spirituality clashes with Miss Leavey's strict beliefs, sparking a heated discussion that brings underlying insecurities to the surface. Rebecca's defense of Simon culminates in tragedy when a perfect pitch during baseball practice ultimately results in a foul ball striking Rebecca, leading to her untimely death.
In the wake of this profound loss, Simon seeks to atone for the incident by gifting Joe his treasured baseball cards, which Joe returns with a stuffed armadillo from Ben. As Joe learns from his grandmother that she is nearing the end of her life, he grapples with the burden of his uncertain future, compounded by the mystery of his father's identity that remains unsolved.
Fueled by Simon's belief that Joe's father is tied to the baseball that caused Rebecca's death, they embark on a quest for answers. Their search leads them to break into their gym teacher's office, only to be met with despair when they find no clues. In a fit of frustration, Joe vandalizes the office, leading the police chief to suggest they attend a children's retreat over winter break as a way to avoid consequences.
With Ben picking them up for ice cream, Simon expresses his deep-rooted belief that he is meant to be a hero, though he remains unaware of what that destiny entails. As the excitement of the Christmas pageant gives way to chaos, fueled by Leavey's misguided creative choices, tensions rise. Simon's sharp criticism results in Russell confiscating his precious baseball cards, leaving him feeling utterly helpless. The disastrous Nativity play, featuring poorly constructed costumes and lyrics gone awry, provides a humorous backdrop as Simon grapples with his dreams. The situation spirals further when Howard's harness gives way, resulting in a comical display of chaos and vomit directed at Leavey, culminating in Simon's suspension and exclusion from the retreat.
As the church prepares for its annual retreat, Simon visits Joe for a poignant farewell before embarking on his mission to reclaim his baseball cards. This quest leads him to discover a shocking truth: the fateful baseball was once carefully stowed away, and it uncovers Russell as Joe's father. Overjoyed, Simon rushes to inform Ben, who drives him to the retreat just as Russell reveals the truth to Joe.
In a shocking twist, their retreat bus crashes into a lake, triggering panic among the children. In a display of courage, Simon takes the lead, guiding his friends, including Joe, to safety while narrowly escaping drowning himself as he rescues the last child. Joe visits Simon in the hospital, marveling at how Simon's small stature became an asset during the crisis. Tragically, Simon does not survive his injuries, leaving Joe heartbroken.
Years later, Joe finds comfort in memories of Simon and is adopted by Ben just before turning 13. The narrative fast-forwards to an adult Joe visiting Simon's grave with his own son, who is named after his unforgettable friend. As they prepare to leave, the younger Simon innocently reminds his father about his upcoming soccer match, and together they drive away into the sunset, bringing their shared story full circle.
