The Further Adventures of Jimmie Dale - Frank L. Packard

In the previous book of adventures, we met Jimmie Dale, a wealthy playboy by day, who at night put on a disguise and became The Gray Seal, a daredevil entering businesses or homes and cracking safes, always leaving a diamond shaped, gray paper "seal" behind to mark his conquest. He never took anything, but just wanted the thrill of it. This had spun out of control when a mysterious woman, whom Jimmie Dale nicknames The Tocsin, caught him at it and blackmailed him into doing her bidding. On her instigation, he got involved in numerous underworld crimes, righting wrongs and protecting innocent bystanders. The Tocsin is herself in hiding from her enemies in the underworld, and the two of them barely escape with their lives when they are pursued by both the criminals and the police.

At the end of those previous adventures, Jimmie Dale thought he had left off his dealings with the underworld for good, and could settle down with the woman he loved. But now, The Tocsin has disappeared. Jimmie Dale is drawn back into the world of crime, crooks and opium dens to look for her, and The Gray Seal is resurrected. (Summary by Anna Simon)