When Elizabeth and Jessica Wakefield invite Nicolas Morrow on a picnic to help cure his blues, their plan has unexpected results. Nicolas falls in love with Barbara, a beautiful girl who involves the twins in their most dangerous mystery yet.
Barbara tells Nicolas they must hide their relationship from the uncle she's visiting. Hearing the fear in her voice, Nicolas 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 serach and leave Barbara in deadly trouble, or save Barbara's life - and risk their lives and Nicolas's!
"I'll call you back in two minutes," Jessica said.
No sooner had Elizabeth hung up with her sister than the phone rang again. She picked up the receiver.
"Boy," she said with a laugh. "That was the shortest two minutes in history."
There was no answer. She could hear heavy breathing on the other end of the line.
"Jess, cut it out," she warned. "This isn't funny."
"I'll tell you what isn't funny," a deep, muffled, sinister voice replied. "What isn't funny is poking into other people's private business. And hanging around where you're not wanted. Am I making myself clear?"
Elizabeth was so frightened she could barely breathe. "Who-who is this?" she managed to ask.
"Listen to me, Miss Wakefield," the man snarled. "I know who you are. That's all that matter's. Just get this through your pretty blond head. We're watching you. We know exactly where you are. And we don't intend to let you get in our way. Do you understand?" He laughed - a dark, aweful laugh - and hung up.
Elizabeth was trembling all over. She was completely alone in the office. The lights had been dimmed when people left hours ago. Who was this caller? What if he was calling from inside the building? What if he trapped her here?
Then to her horro, that phone rang again.
After seven rings, she picked it up. "H-hello?" she gasped.
"Liz, where were you?" Jessica demanded. "Didn't I tell you it was only going to before I rang you back?"
"Yeah," Elizabeth mumbled, looking around the dark office. She fought hard not to scream. "Jess," she said as calmly as she could. "I'm coming home right away. If I don't get there in fifteen minutes, call the police."