- For the original version of this book, see What Happened at Midnight (original text).
What Happened at Midnight is the tenth volume in the Original series. It was first issued in 1931, but the text and artwork were revised and updated in 1967. The book was published by Grosset & Dunlap and produced by the Stratemeyer Syndicate.
Frank and Joe Hardy are given an odd assignment by their father - to burgle the house of his client, at midnight, and retrieve a valuable new invention. They are interrupted in their endeavours by a pair of thieves but manage to fight the intruders and flee with the invention. The next night, while trying to chase off another thief intent on stealing the mysterious device, Joe is kidnapped and Frank and his pals must find him before he is taken overseas and lost forever. They look and look at the caves and they don't find him in the previovs caves then at the very top they see a cave that looks very small but they climb up the step slope and they find Joe very beat up.
- "Anchor Pete", a sailor and smuggling gang member
- Dr. Bates, the Hardy family physician
- Ben, a smuggling gang member
- Mr. Bickford, owner of Bickford's jewelry shop
- Larry Burton, a radio ham
- Chris, a smuggling gang member
- Harbor Master Crogan
- Beef Danion, crewmember of the Rizzolo (mentioned only)
- Detective Lieutenant Danson of the NYPD
- John Gross, a smuggling gang member
- Hank, a sailor on the Annie K
- Cousin Helen, a cousin of Fenton and Gertrude Hardy who lives in Gresham (mentioned only)
- Mrs. Hooper, Biff's mother
- Elmer Hunt, president of the Oceanic Electronic Company (mentioned only)
- Agent Emery Keith of the FBI
- Agent George Mallett of the FBI
- Marcy, a sailor on the Annie K
- Taffy Marr, leader of the smuggling gang
- Torchy Murks, a petty thief and pal of Chris'
- Hank Olsen, a farmer
- "Shorty", a smuggling gang member
- Captain Volper of the tugboat Annie K
- Cole Weber, president of the Central Antique Airplane Club
- Malcolm Wright, an inventor
- New York City
- Rocky Point, on the north shore of Barmet Bay
Businesses and organizations
- The Central Antique Airplane Club
- The Federal Aviation Agency (mentioned only)
- The Oceanic Electronic Company, San Francisco (mentioned only)
- The Annie K, a tugboat
- Frank and Joe's convertible
- The Rizzolo, a ship used by the smuggling gang (mentioned only)
- The Sleuth
- The Water Devil, a whaler
- The Hardys have devised a series of hand-squeeze signals;
- "One hard squeeze meant, 'Let's attack!' Two indicated caution. Long, short, long meant, 'We'd better scram.' An ordinary handshake was, 'Agreed.'"
- — The Hardy Boys #10 - What Happened at Midnight p7
- The Hardys have a private short-wave radio set in Fenton's study.
- The revised text of What Happened At Midnight was issued in Great Britain as the 21st volume in the Collins series.
- During the 1970s, What Happened At Midnight was published in Israel using the revised Grosset & Dunlap cover artwork. However, starting in 1992, the cover art was changed to a Norman Rockwell painting of an early 20th century family sitting around a table in prayer. For both the 1970s and the 1990s versions the internal artwork was the same as the Grosset & Dunlap edition.