Richard Thorncroft, once the charming Detective Bruce Mindhorn from a beloved 80s television series, finds himself in a precarious position, hustling to sell low-quality products. Nostalgic for his former glory, he seizes a chance to revitalize his career by returning to the Isle of Man, the iconic filming location of Mindhorn, to assist the police with a murder investigation tied to an escaped mental patient named Paul Melly. However, the local authorities are skeptical of Richard's abilities, seeing him as merely an arrogant remnant of a bygone era.
As Richard tries to navigate the complicated relationships and rivalries from his past, he realizes that his Mindhorn legacy is still alive for some. His former love interest, Patricia, is now partnered with his old stuntman, Clive, and they share a daughter, Jasmine. On the flip side, another cast member, Pete Eastman, has enjoyed considerable success with a spin-off series, deepening Richard's feelings of being a faded star.
A pivotal meeting with Moncrieff, his ex-manager, gives him a glimpse of redemption and rekindles his passion for Mindhorn. However, after a wild night filled with partying and drugs, Richard faces a harsh reality: he might be out of the game entirely, particularly when his agent drops him.
While waiting for his ferry back home, Richard is startled by unsettling fan mail from Melly, which includes a mysterious videotape hinting at the mayor's potential involvement in the crime. Moncrieff, always on the lookout for personal gain, suggests they use the tape to blackmail the mayor, but Richard declines, opting instead to confront Melly and alert the authorities. To his shock, Richard soon finds out that Moncrieff has swapped the crucial tape for a fake version.
The tension escalates when Moncrieff is murdered by DS Baines, the mayor's shrewd niece, who is entangled in the conspiracy. With his ally gone, Richard forms an unlikely partnership with Melly to expose the corrupt officials and reveal the truth. Their escape from the police unfolds in a vintage car loaded with Mindhorn merchandise and DIY spying gadgets. Richard dons an extravagant Mindhorn outfit, embracing his inner detective to solve the murky puzzle involving deception and corruption that threatens their lives.
As Richard approaches Patricia's home, he finds their cherished vehicle commandeered by a local parade. In a surprising twist, Clive uncovers a secret stash of heartfelt letters from Richard, hidden away. Joined by Melly, Richard and Patricia unite in a spirited quest to reclaim their car, but their escapade is hampered by the relentless pursuit of DS Baines. A tense car chase leads to a chaotic confrontation on the beach.
As calm returns, Richard's worst fears materialize: Melly's improvised "copy" tape is revealed to be an artfully crafted model made from plasticine. The group's fragile peace is obliterated when Melly is shot, seemingly sealing his fate. DS Baines arrives, unleashing chaos and seemingly dealing a fatal blow to Patricia, leaving Richard in a state of despair.
Nevertheless, this dire situation takes an unforeseen twist when Richard uses Melly's Mindhorn recorder belt, which holds vital evidence—DS Baines' own confession. With the police closing in, Richard and Patricia cleverly orchestrate events to expose the duplicitous duo, ultimately revealing the mayor's and Baines' roles in the murders.
Restoration of order finally occurs; Richard and Patricia share a heartfelt reunion while Baines and the corrupt mayor face justice behind prison bars. In a shocking conclusion, Melly is discovered alive in his hideout, leaving Richard both shaken and thankful for their miraculous escape.
