Ben purchases a paddle-wheel steamboat which delivers freight on Lake Tahoe. A criminal mastermind named Jameson Fillmore gets wind of this and, with the help of his gang (including Brother Guy, a phony preacher) plans to use the boat to aid in his escape after stealing $1 million cash Ben is expecting for payroll.
Fillmore introduces himself to the Cartwrights as the owner of a lumber enterprise interested in purchasing some timber from the Ponderosa. Candy is suspicious and plans to infiltrate the gang by posing as an associate interested in getting a cut of Fillmore's haul.
Meanwhile, we see the gang's criminal activities at work: Brother Guy blocks the road and deliriously rants about being "saved" by the Lord, before his lackeys loot the unfortunate stagecoach and kill their innocent passengers.
Fillmore, meanwhile, corners the steamboat captain, Nells Larson (a former cellmate and reformed criminal who just wants to move on with his life). Afraid that Larson might reveal to the Cartwrights his true background, Fillmore kills him. Larson's daughter, Julie, finds her father soon afterward.
Candy rides back to camp, where Ben notices the payroll pouch has been opened. Candy explains his plan and, although Ben is not pleased that he was not consulted beforehand, he allows Candy to have Fillmore accompany him during the haul – both of them unaware that Fillmore plans to hijack the steamboat, take the $1 million for himself and escape to San Francisco.
After Fillmore's gang take over the steamboat and throws the crew off the ship, the boat takes off. Julie is hiding on board and plans to confront Fillmore, but he and his lackeys find her first. Candy attempts to steer the boat back to shore, but is knocked unconscious by the bad guys.
Fillmore holds Julie hostage in the wheelhouse, but while his back is turned, she pulls a gun on him and fatally shoots him. She is able to run the steamboat aground, where the Cartwrights are able to win a gunfight with the thieves. Candy, meanwhile, regains conciousness and traps Brother Guy, who begs forgiveness but is hiding a tie-down hook in his fist; Candy is too quick, however, and defeats the faux preacher.