Colonel March intercepts Mrs Pollard, a wealthy widow who is hiding a gun.
She had a fling with a romeo Frenchman who after getting bored. Walked out on her and took some jewelry with him.
Mrs Pollard has come to London after she spotted him as part of the French fencing team. She wants to know if he really was a cad. Colonel March is suspicious that the fencer is a serial romancer of older women and might have left one of them dead.
When Mrs Pollard is later found dead, Colonel March is convinced that it is not suicide.
It is a dreary story despite some droll observations from Inspector Goron of the Surete.
There is a lot of interrogation of the French and British fencers. This is where Colonel March realise that one of them has given themselves away. There is no sitting on the fence from Colonel March.