Tony Blake, a young writer struggling to find his place in the artistic world, faces eviction from his rented room in a Chelsea house shared with other creatives, including the eccentric abstract painter Arnold. Desperation leads Tony to devise a clever plan: he wants to generate interest in his rejected book about the Loch Ness Monster by creating a fake sighting. Along with his friend Vernon, who is into electronic music, Tony constructs a convincing fake monster. They capture photographs of the creature in the Serpentine, causing a local tramp to have a startling encounter.
Eager to take their hoax further, Tony, Vernon, and Vernon's somewhat clueless girlfriend, Charlotte "Charlie" Pinner, decide to head to Scotland. They travel in a second-hand Rolls-Royce hearse, adding a touch of eccentricity to their trip, and along the way, they pick up a young French hitchhiker named Marie. Their journey is also interrupted by Charlie’s dipsomaniac father, who is determined to follow them.
When they arrive at a cozy Loch Ness inn, they quickly become part of the local scene. The inn’s landlord, Harry Sutton, is busy hiding dozens of poached salmon from two local policemen, adding to the humorous chaos. Tony quickly befriends the local postman and other villagers, who become increasingly convinced that the monster might be real—especially after Vernon secretly installs a speaker that emits a terrifying roar, further promising that a genuine monster is nearby.
The next day, the inn fills with curious tourists and eager press, much to the delight of Sutton and the townsfolk. However, the excitement starts to fade when Tony fails to deliver a promised photo of the monster, casting doubt on the entire ordeal. Meanwhile, Vernon and Charlie decide to tie the knot, and Marie inexplicably falls for Tony, creating a complex web of romantic entanglements amidst the hoax. In a comical twist, the poached salmon are accidentally loaded into a police car, adding to the chaos.
In an effort to keep up the illusion, Tony and his friends secretly build another fake monster for a photo opportunity. To their surprise, they discover that several locals had the same idea—creating their own fake monsters to attract visitors and boost the area’s economy. The locals also uncover the concealed speaker that Vernon used to broadcast the monstrous roars, realizing they’ve been duped all along.
With tensions rising, an angry mob chases Tony and Marie, who attempt to escape by rowing across the loch. Just as they think they might be safe, the climax unfolds as the real Loch Ness Monster unexpectedly rises from the water, revealing that the legend might not be as fabricated as everyone believed. The film concludes with this dramatic and humorous revelation, leaving the audience pondering whether the monster is truly a myth or something more tangible lurking beneath the surface.
