As CBS cameraman Harold "Harry" Hinkle (Walter Matthau) navigates the lively chaos of Municipal Stadium during a Cleveland Browns game, he unexpectedly finds his fate tangled with that of Luther "Boom Boom" Jackson (Jack Lemmon). A sudden collision leaves Harry with a minor yet troublesome injury, which sparks a clever scheme from his lawyer brother-in-law, William H. "Whiplash Willie" Gingrich. Gingrich orchestrates a plot to feign Harry's partial paralysis affecting both his leg and hand, setting the stage for a hefty insurance payout.
Though initially hesitant, Harry's desire to rekindle a romance with his ex-wife, Sandy, pushes him to go along with the plan, hoping that his new "invalidity" will draw her back into his life. However, the legal team from O'Brien, Thompson, and Kincaid grows suspicious about the legitimacy of Harry's condition. Their medical experts, initially misled by the traces of a past injury—specifically a compressed vertebra—and Harry's behavior altered by novocaine injections from a paroled dentist in league with Gingrich, waiver on their verdict.
The only skeptic among them is Professor Winterhalter, who continues to assert that Harry’s condition is a sham. To unearth the truth, O'Brien, Thompson, and Kincaid employ the sharpest private eye in Cleveland, Chester Purkey, to keep a close watch on Harry. Anticipating their move, Gingrich warns Harry about their surveillance and discourages any romantic interactions with Sandy. Additionally, he introduces the idea of the "Harry Hinkle Foundation," a non-profit charity designed to secure a significant chunk of any insurance settlement.
Sandy's reappearance in Harry's life ignites hope, but she is soon privy to Gingrich's scheme. He confesses to her in a private conversation that the foundation is nothing more than a tactic to leverage the insurance company. With confidence in the scheme, he guarantees there will be ample funds for everyone involved. As Harry relishes in having Sandy by his side again, he grapples with the guilt-ridden demeanor of Boom Boom on the field, facing crowd jeers and an eventual grounding from the team after a barroom brawl.
Harry’s frustrations escalate as he learns that Gingrich is too engrossed in negotiating a deal with O'Brien, Thompson & Kincaid for Boom Boom, ignoring Harry’s interests in the process. Disillusioned, Harry realizes that Sandy’s motivations for returning are inherently financial. Despite this, fortune seems to smile on Harry when he lands a substantial $200,000 settlement for his supposed injuries.
However, just as Harry’s luck appears to change, Purkey devises a cunning plan to bring the whole scheme to light, setting his sights on Harry's apartment. Disguised as a casual visitor to collect equipment, Purkey begins an offensive tirade against Boom Boom, which causes Harry to snap. Overcome with indignation, Harry rises from his wheelchair and delivers a stunning punch to Purkey’s jaw, leaving the aspiring videographer scrambling to capture the moment on film amidst complaints of poor lighting.
Not one to back down, Harry invites Purkey to record the encounter again, this time in better light, and with some guidance on camera settings from his formidable lawyer. The outcome remains unchanged: Purkey receives another wallop to his face, leading to an exhilarating display of wheelchair gymnastics involving bedsprings and curtain rods.
As Harry prepares to exit, an accidental interaction causes him to knock Sandy off balance while she searches for a misplaced contact lens. Meanwhile, Gingrich, blindsided by Boom Boom's deceit, vehemently expresses intentions to take legal action against the insurance lawyers for invading their privacy and pledges to report Purkey’s derogatory remarks to various organizations.
Upon arriving at the stadium, Harry encounters Boom Boom on the verge of quitting football for a career as “The Dark Angel,” a professional wrestler. With infectious enthusiasm and a shared love of the game, Harry manages to invigorate Boom Boom’s spirit, leading the two to an impromptu game of catch that reignites their passion for gridiron challenges.
