Franks and Joe Hardy are enjoying time off when the two new boys intown, Greg and Mike Rawley, convince them to investigate their stepfather, Walter. Greg and Mike suspect him of plotting to kill their mother, Linda. But there's a hitch: Walter Rawley is one of Fenton Hardy's oldest friends and an upstanding citizen of Bayport.

With the Rawley boys' help, the Hardy brothers check out the contents of Linda Rawley's safe, looking for the diary of Walter's first wife, which contains the murderous evidence. Then Linda is kidnapped. Using themselves as bait, the Hardy boys flush out the kidnappers -- only to find they are mere pawns in an elaborate scheme against their father's old friend.