Commissioner Duval is investigating the disappearance of Hotel Director Blanche. What he does not know: the kidnappers mistook the young lady for a wealthy heiress and she was taken to a ship anchored three miles from Marseille. The police find this ship strange anyway, but they are not allowed to intervene outside the three-mile zone. Therefore, Inspector Lacroix agrees to board the ship as a tourist. But it doesn't work out and Lacroix won't return...
