"Stay away from the Witch! It's a jinx boat!" Such a grim warning would surely frighten a less courageous girl than Kay Tracey. To Kay, whose skill as an amateur detective is already well known in her home town, the threat is a challenge that cannot be ignored. She must find out why someone is trying to keep her from going aboard the Witch!

Defying the jinx, Kay and her friends the Worth twins, peppy Betty and studious, quiet Wilma, board the trim little houseboat for a summer trip down the Sharon River. Sailing close behind and always mooring near them, is the strange black Ajax, guarded day and night by a vicious watchdog. Two mysterious, unfriendly men are aboard, armed with guns and wary of any boat approaching them.

At night, from the darkened Ajax, a flashlight winks a message in Morse code and is answered by beam from the shore. Can there be any connection between this sinister gang and the recent jewel robberies the police have been unable to solve?

Kay, Wilma, and Betty find their holdiay cruise has turned into a precarious chase. The trail finally leads the girls to a dangerous spot from which there seems to be no escape...