When an engine or weapon misfires, it doesn't function or ignite correctly. When a plan misfires, it doesn't work quite the way you imagined it would.
The original meaning of misfire is gun-related; when pulling a trigger doesn't result in the gun firing, the gun misfires. Around the turn of the 20th century, the word began to be used for combustion engines that couldn't get started because their fuel wouldn't ignite. Figuratively, misfire is good for describing ideas that don't ignite: "I'm afraid my plan to talk my parents into taking us to Disney World may misfire in the end."