Aristide Valentin, the Chief of Paris Police, was having a party at his house in the garden. His guests arrived before he was able to. All of the guests were outside in the garden. There was a large wall surrounding the garden and there weren't any exits. The only way to enter or exit the garden was from the house and by the front door. A murder was committed outside of the garden. This seemed like an impossible situation because of not having any exits from the garden, but was it?
The body was Lord Galloway. The man was dressed like he was going to their party, but he hadn't been there. Who wanted him dead? Who had the opportunity to murder him? After Valentin arrived, he wanted everyone to stay all night until the crime was solved. Now, Father Brown, who was the detective solving the case, had to figure out how the body got there in the grass since there weren't any gates to the garden, and he wasn't inside the house?
Moreover, there was another murder committed and one person, Mr. Brayne, left the house. This shocked Mr. Valentin. The suspense continues and the end is surprising.