"What Happened at Midnight" The Hardy Boys by Franklin W. Dixon

Frank and Joe race against time to retrieve a scientist’s top-secret invention before thieves get their hands on it. But the young detectives soon discover that they are involved in a mystery far greater in scope than just safeguarding the invention. When Joe is kidnapped, this incident starts Frank of on a chase that could end disastrously for him and his pals.

Believed to be a 1972 A-70 printing