The title is a play on "hair trigger", referring to any weapon or other device with a sensitive trigger.
After opening credits underscored by an instrumental of "Cheyenne", an old-fashioned train is seen rolling along through a western desert. It passes another train going around a utility pole, and voices are heard repeating "Bread and butter".
Bugs is riding in the mail car of a train, singing a nonsense song called, when a pint-sized bandit attempts to rob the train (with the underscore playing stereotypical "villain music"), only to have it pass clear over his head. He then calls for his horse, which he needs a rolling step-stair to mount. He catches up and boards the train and begins to rob it while the mail clerk wraps himself in a package marked DON'T OPEN 'TIL XMAS. The bandit accidentally throws Bugs Bunny in his sack. Bugs assumes he's Jesse James. The bandit scoffs and tells him (and the audience) who he actually is: "I'm Yosemite Sam, the meanest, toughest, rip-roarin'-est, Edward Everett Horton-est hombre what ever packed a six-shooter!" (This pattern of Sam introducing himself to Bugs and the audience would continue in other cartoons.) Bugs tells Sam that there is another tough guy in the train packing a "seven-shooter", and Sam goes looking for him – unaware that he is actually Bugs in disguise.
Various fights ensue, as each character temporarily gets the upper hand for a while. After one such skirmish, Bugs tricks Sam into dashing into a lounge car in which a horrific fight is occurring, actually stock film footage of a stereotypical western saloon fight. With the sounds of crashes and bangs in the background, Bugs calmly sings "Sweet Georgia Brown" to himself. Sam emerges tottering, banged and bruised, to a comical instrumental of "Battle Cry of Freedom", and a race-based gag occurs that is subtle enough it is usually left intact in network showings: Bugs effects the stereotyped voice of an African-American train porter, and has the dazed Sam convinced he's supposed to disembark the train, piling him up with luggage; Sam even hands Bugs a silver coin as a tip, and Bugs says, "Thank you, suh!" As Sam steps off the moving train, the mail-drop hook grabs him and temporarily whisks him off the train. Bugs, thinking he has vanquished Sam, yells "So long, screwy, see ya in Saint Louie!" a line that would be echoed in Bugs Bunny Rides Again and A Feather in His Hare. But Sam gets back on board somehow, and attacks Bugs on the roof.
Finally, Sam has Bugs tied up, dangling from a rope, weighted down by an anvil, and fiendishly cutting through the rope, while the train is passing over a gorge. The screen fills with the words the narrator (also Mel Blanc, in pretty much his natural voice) is saying, "Is this the end of Bugs Bunny? Will our hero be dashed to bits on the jagged rocks below?" and so on. Then Bugs walks across the screen, dressed in top hat and tails, carrying a bag full of gold (reward money), and dragging the tied-up villain behind him, mocking the on-screen words ("Is he to be doomed to utter destruction? Will he be rendered non compos mentis?"). Bugs closes by turning to the audience and repeating a popular radio catch-phrase from Red Skelton's "Mean Widdle Kid": "He don't know me vewy well, do he?" as a bar of Kingdom Coming plays on the track at iris-out.