"Duddy" Kravitz (Richard Dreyfuss) is a brash, restless young Jewish man growing up poor in Montreal, Canada. His taxi driver father Max (Jack Warden) and his rich uncle Benjy (Joseph Wiseman) are very proud of Duddy's older brother Lenny, whom Benjy is putting through medical school. Only his grandfather (Zvee Scooler) shows the motherless Duddy any attention.

Duddy gets a summer job as a waiter at a Jewish resort hotel in the Laurentian Mountains. His hustle, energy and coarse manners irritate condescending college student and fellow waiter Irwin. Irwin gets his girlfriend Linda, the daughter of the hotel's owner, to persuade Duddy to stage a clandestine roulette game. Unbeknownst to Duddy, the roulette wheel is crooked, and he loses his entire $300 earnings to Irwin and some hotel guests. Fortunately for Duddy, the other waiters find out and make Irwin give back the money. Unaware of this, the hotel guests, led by Farber, feel bad and give him a further $500.

Duddy starts a serious relationship with another hotel employee, French-Canadian Yvette (Micheline Lanctôt). One day, she takes him on a picnic beside a lake. Duddy is stunned by the beauty of the setting, and his ambition crystalizes: taking to heart his grandfather's maxim that "a man without land is nobody", he decides he will buy all the property around the lake and develop it. Because the current owners might not want to sell to a Jew, he gets Yvette to front for him.

Duddy sets out to raise the money he needs. He hires blacklisted, alcoholic American director Friar (Denholm Elliott) to film weddings and bar mitzvahs. His first customer is Farber, who drives a hard bargain. If he does not like the result, he will not pay. Despite Friar's artistic pretensions, the film is a success, and more orders are quickly forthcoming.

However, when a piece of land comes up for sale, Duddy does not have enough money. He begs his father to get him an appointment with his friend Dingleman, "the Boy Wonder", a rich, successful gangster who had equally humble beginnings. Dingleman turns down his request for a loan, but later invites him to discuss his scheme on a train to New York. It turns out that Dingleman just wants a dupe to (unknowingly) take the risk of smuggling heroin, but Duddy gets a loan out of it.

On the train, Duddy meets good-natured Virgil (Randy Quaid) and, ever open to a deal, offers to buy his pinball machines, which are illegal in the United States. When Virgil shows up, Duddy does not have enough money to pay him, so he hires Virgil as a truck driver, even though he is an epileptic. Tragedy strikes when Virgil has a fit while driving and crashes; he is left permanently paralyzed from the waist down. Duddy is distraught and guilt-ridden. Blaming Duddy, Yvette leaves him to care for Virgil.

Duddy becomes alarmed when Dingleman finds out about his lake. When the last piece of property Duddy needs comes on the market, Dingleman bids for it. Desperate, Duddy forges Virgil's signature on a check to buy the land, leading to a final rupture with Yvette and Virgil.

Undeterred, Duddy proudly takes Max, Lenny and his grandfather to see his property. When Dingleman shows up to offer to raise the financing for its development, Duddy tells him to get off his land. However, Duddy's grandfather refuses to pick out a plot for his farm; Yvette has told him what Duddy did to get it.