"Tell Father to send help --" The frantic call sends Kay dashing to the home of her friend Rosa Lusardi. When she pushes open the door an astonishing scene meets her eyes. Rosa Lusardi sits bound hand and foot to a chair by the telephone table. She has managed to work the gag free of her mouth and has knocked the telephone from its cradle so she can call Kay. Her pet dog, badly beaten, lies in a corner of the room. And her father's valuable violin -- a Stradivarius -- has been stolen!

Kay Tracey's determination to track down the heartless thief leads her and her friends, Betty and Wilma Worth, into one exciting adventure after another. At one point Kay's eagerness backfires when she fails to get off a departing ship and finds herself hauled before the captain as a stowaway.

Later, when Kay unearths a key bearing the mysterious number 777, she feels that at last she has found the real clue to an unusual secret. Ethel Eaton, Kay's jealous and egotistical classmate at Carmont High, has other ideas and puts Kay in real danger in her attempt to put her rival off the track. Kay must find her way out of the maze of intrigue to the solution of the mystery.