The narrator is sent by a friend to interview an old man, Simon Wheeler, who might know the location of an old acquaintance named Leonidas W. Smiley. Finding Simon at an old mining camp, the narrator asks him if he knows anything about Leonidas; Simon appears not to, and instead tells a story about Jim Smiley, a man who had visited the camp years earlier.
Jim loves to gamble and will offer to bet on anything and everything, from horse races to dogfights to the health of the local parson's wife. He catches a frog, whom he names Dan'l Webster and trains to jump over the course of three months. When a stranger visits the camp, Jim shows off Dan'l and offers to bet $40 that he can out-jump any other frog in Calaveras County. The stranger, unimpressed, says that he would take the bet if he had a frog, and Jim goes out to catch one, leaving him alone with Dan'l. While Jim is away, the stranger pours lead shot down Dan'l's throat. Once Jim returns, he and the stranger set the frogs down and let them loose. The stranger's frog jumps away while Dan'l does not budge, and the surprised and disgusted Jim pays the $40 wager. After the stranger has departed, Jim notices Dan'l's sluggishness and picks the frog up, finding it to be much heavier than he remembers. When Dan'l belches out a double handful of lead shot, Jim realizes that he has been cheated and chases after the stranger, but never catches him.
At this point in the story, Simon excuses himself to go outside for a moment. The narrator realizes that Jim has no connection to Leonidas and gets up to leave, only to have Simon stop him at the door, offering to tell about a one-eyed, stubby-tailed cow that Jim had owned. Rather than stay to hear another pointless story, the narrator excuses himself and leaves.