The title is a play on "unruly hair", a synonym for "wild hair", which was the pun on the first Bugs Bunny picture, The Wild Hare.
At one point, Elmer has Bugs at gunpoint, the barrel of the shotgun poking his chest. Bugs says, "Only a rat would shoot a guy... (turns around) ...in the back!" After some more taunting, Elmer fires at point blank range, obscuring Bugs in a cloud of gunsmoke. Elmer turns to the camera and says, "So I'm a big fat wat!" Bugs suddenly appears through the cloud, unharmed, and effects a Jerry Colonna-like schtick: "Aaah! Have some cheese, rrrat!" and stuffs a large wad of cheese into Elmer's mouth before scampering off.
At the climax, their antics manage to set off TNT in the supply store, and the explosion miraculously lays the ties and the track, followed immediately by an engine in full steam.
Bugs is riding away from Elmer at the back of the train, waving goodbye. He turns to the audience with a suddenly startled look, and leaps from the train, crashing and screeching to a halt. He then stands up, brushes himself off, and in a stark reminder of the still-raging World War II (the cartoon was released in February, a couple of weeks before the raising of the flag on Iwo Jima), tells the audience, "None of us civilians should be doing any unnecessary traveling these days!" He then walks off down the tracks with a pole and a knapsack over his shoulder, into the moonlight, accompanied by an instrumental bar of "Kingdom Coming". Iris out.