During a break in, a burglar finds something he wasn't expecting.

The theft of Maigret's revolver from his home begins a detective story that leads to Maigret travelling from Paris to London to find the young man who stole it, and the woman who is in danger of being his victim.

Professor Gouin, a brilliant surgeon, but a man that 'does not know what makes people tick' is caught in the drama of murder.