"Frank and Joe are enjoying some time off when the two new boys in town, Greg and Mike Rawley, convince them to investigate their step-father, 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 that they are mere pawns in an elaborate scheme against their father's old friend."