It's great to take part in a real carnival! The joy of the four young detectives knows no bounds. But in the midst of the fun, a girl in a mask approached Arthur, handed over some strange package and promised... not to tell the police anything. What is this, a prank? Just a mistake? Or maybe part of some criminal plan? The guys decide to look for the mysterious stranger and find her. This is where the fun begins...

The story was also published under the title “The Secret of the Carnival Costume.”