Fred, a raffish safe blower, takes refuge in the Paris Metro after being chased by the henchmen of a shady businessman from whom he has just stolen some documents. While hiding out in the back rooms and conduits of the Metro, Fred encounters a subterranean society of eccentric characters and petty criminals.

Paris, 1910. Emile, a shy movie projectionist, and Raoul, a colourful inventor, find themselves embarked on the hunt for a monster terrorizing citizens. They join forces with Lucille, the big-hearted star of the Bird of Paradise cabaret, an eccentric scientist and his irascible monkey to save the monster, who turns out to be an oversized but harmless flea, from the city's ruthlessly ambitious police chief.

Mr. Takazawa, an elderly invalid who is cared for at his home by Haruko, a young nursing student, is chosen by the Japanese Ministry of Public Welfare to test the Z-001, a computerized hospital bed with robotic features that allegedly displays more efficiency and skills than any human nurse, but Haruko mistrusts a machine unable to consider human feelings.