Old Timer is fed up with being swindled by Blacknail the banker, and robbed by Mo the local bandit. He asks his friend Lucky Luke to escort his gold in a self-designed armoured wagon. The new wagon resists all attacks! Soon, every prospector in Nugget Town has entrusted their gold to Luke and his wagon. The bandits, deprived of a living, set up a trade union, and lie down in the wagon's path in a protest demonstration… A wind of rebellion blows across town, and an unwilling Luke is obliged to negotiate.