CHAPTER XX

A Note of Warning Three days later, Fenton Hardy, who had been away from home on business, received a note. No one saw the man who left it at the door. The Hardy boys were at school and Mrs. Hardy was busy in the kitchen. She heard the front doorbell ring and went to answer it. But when she opened the door there was no one in sight. She looked out and saw a man walking briskly down the opposite side of the street. A woman with a baby-carriage was strolling past the house, and farther down the street two men were st…