In the movie The Big Mouth, the fisherman named Gerald Clamson, played by Jerry Lewis, is on a fishing trip when he comes across a gang of diamond smugglers. One of the gangsters, named Joseph "Mister" X, played by Jerry Lewis in a dual role, has a striking resemblance to him. When Mister X is injured in a car accident, Gerald takes his place at a resort hotel where the gangsters are planning to sell their stolen diamonds.
At the resort, Gerald meets a young woman named Suzie Cartwright, played by Susan Bay, who has come to the resort to pick up her father's car. The two quickly fall in love, but their romance is complicated by the fact that Gerald is impersonating Mister X and is being pursued by the gangsters. Meanwhile, the real Mister X, who has been hiding out at the resort, is trying to clear his name and reclaim his stolen diamonds.
As the plot unfolds, Gerald and Suzie team up to outsmart the gangsters and prove their innocence. The movie features a mix of slapstick comedy, action, and romance, as well as several musical numbers performed by Jerry Lewis. Overall, The Big Mouth is a classic 1960s comedy caper that showcases Jerry Lewis's unique brand of humor and talent.