Quinquin has now come of age and is now nicknamed Coincoin. He hangs out on the seacoast of the Côte d'Opale and attends meetings of a nationalist party with his childhood friend L'gros. His old love Eve has abandoned him for Corinne. When he finds a strange kind of liquid near his town, the residents suddenly start to behave very strangely. Our police heroes, Van der Weyden and his loyal assistant Carpentier investigate these alien invasions.