Commissaire Perrache, the chief of the D.S.T., the French domestic intelligence service, tries to neutralize a group of terrorists named the Partisans de la Métropole. He sends one of his best agents, inspector Louis Vignon, incidentally the husband of his charming secretary Lucette, on the trail of an illegal transmitter. But Vignon gets kidnapped by the gang and held captive on a boat off the shores of Monaco...