Brothers Adolph and Arthur Caille are mechanical geniuses. They've just created one of the first slot machines and have presented it to their local senator, a notorious gambler. Much taken with the device, he agrees to send them on a tour of American cities to test the machine's popularity. And, to escort them on this dangerous journey, he calls on his old friend Lucky Luke... So begins the wacky tale of how the one-armed bandit conquered the West!