When Elizabeth and Jessica Wakefield invite Nicholas Morrow on a picnic to help cure his blues, their plan has unexpected results. Nicholas falls in love with Barbara, a beautiful girl who involves the twins in their most dangerous mystery yet.
Barbara tells Nicholas they must hide their relationship from the uncle she's visiting. Hearing the fear in her voice, Nicholas decides to tell the twins, and they start investigating. The more they find out, the more desperate Barbara's situation seems.
Then Jessica and Elizabeth are threatened by an anonymous caller. Now they face a terrible choice: either give up their search and leave Barbara in deadly trouble, or save Barbara's life -- and risk their lives and Nicholas's!