Octave Verbrugge, a powerful Belgian building contractor, met Hugo Saint-Laurent, a brilliant French engineer, during a business trip and made friends with him. Now that his bureau of expertise is experiencing difficulties with one of their important projects he has the idea to hire his friend to help resolve the problem. Hugo agrees and stays as a guest at Octave's home in Ghent, where the promoter lives with his two unmarried sisters, Françoise, a beautiful but mysterious woman, and Aurélie, a very strict and withdrawn lady.