Young Kay Harker has a variety of adventures in search of the truth about a famous treasure that his great-grandfather, a merchant captain, was given for safe-keeping then lost when his crew mutinied. He is aided in his quest by the Midnight Folk, an association of Talking Animals, Living Toys, and other fantastic creatures.
But not all the creatures that haunt the night are friendly: a coven of witches is also after the treasure, led by the scheming Abner Brown and the sinister Mrs Pouncer, and woe betide anyone who gets in their way.
A sequel, The Box Of Delights, was published in 1935. The two books also have links, in terms of shared settings and characters, with a series of adventure stories for adults which began with Sard Harker in 1925.
This novel provides examples of:
- All Witches Have Cats: Blackmalkin and Greymalkin serve the coven as familiars. Nibbins also used to be a familiar, but is now a household cat who sides with Kay.
- Brick Joke: Kay's letter-writing style, which his governess scolds him about during a writing lesson early in the book, returns full-force near the end when he writes a letter to the authorities to let them know the treasure has been found.
- Character Overlap: Abner Brown previously appeared as a henchman of the villain (also a black magician) in Sard Harker. Sard Harker himself is perhaps a relative of Kay, although they're not explicitly connected (the details that are given are at least sufficient to establish that he's not a direct ancestor).
- Deserted Island: What with all the mutinies and maroonings, the history of the treasure includes several, each more bleak and inhospitable than the one before.
- Direct Line to the Author: The "Where Are They Now?" Epilogue includes a couple of lapses into first-person, indicating that the author lives in the neighbourhood where the novel is set and has met (or, in the case of the fox, had his poultry raided by) some of the characters.
- Flying Broomstick: Used by the witches.
- Gossipy Hens: Kay's governess's social circle includes a flock of them, with names like Mrs Gossip, Mrs Tattle, and Mrs Scatternews.
- The Highwayman: Kay is told a tale about Benjamin the highwayman, who used to live in the area.
- Latin Land: Santa Barbara, where the treasure came from.
- Living Toys: Kay's toys are among the Midnight Folk of the title.
- The Mutiny: Captain Harker's crew mutinied and marooned him so they could steal the treasure. The subsequent history of the treasure turns out to involve several more mutinies and maroonings with the same motivation.
- Narrative Profanity Filter: Whenever Roper Bilges uses a profane verb, which he does often, it's obscured by verbing the nearest relevant noun.'It sounded like a young jackdaw got down the chimney again.'
'I'll jackdaw them jackdaws one of these days,' he said, 'if they keep on jackdawing me.'
- Oracular Head: The coven uses a Brazen Head that can see into the past in an attempt to locate the treasure.
- Or Was It a Dream?: Played with all through the book. All the supernatural events end with Kay waking up, but the "mundane" adventure story (where he's definitely awake) only makes sense if the things he learns on these expeditions are true.
- Parental Abandonment: Kay's parents are both dead.
- Portrait Painting Peephole: Kay uses one to spy on a meeting of the coven.
- Public Domain Character: King Arthur and his court make a cameo appearance.
- Seven League Boots: The witches have seven-league boots, as well as forty-nine league boots.
- Species Surname: Kay's toys include a dog named Dogg and another named P. Dogg (they're said to be cousins), as well as a bear named G. L. Brown Bear. Many of the wild animals he meets also seem to be named after their species, as Bat, Otter, Water Rat, etc.
- Talking Animal: Many of the Midnight Folk are these.
- Two Aliases, One Character: Kay's governess Sylvia Daisy and the wicked witch Mrs Pouncer are revealed to be one and the same.
- When I Was Your Age...: Kay's governess and Mrs Tattle have a session of complaining about what young people these days are coming to, and how none of them are "what we were when we were girls". "Which," the narrator drily notes, in the case of Kay "was very likely true."
- "Where Are They Now?" Epilogue: Though not marked off as an epilogue (the book has no chapter divisions), the last couple of pages consist of a series of descriptions of what happened to various characters afterward.
- Wicked Witch: Mrs Pouncer and her colleagues, complete with familiar felines, flying broomsticks, tall pointy hats, wrinkled faces, hooky noses, etc. It turns out that the faces are cunningly-fashioned masks that come off with the hats when they return to their respectable daylight lives.
- Your Size May Vary: The relative sizes of Kay and the various Midnight Folk is never nailed down, and seems to vary according to convenience. Whenever Kay is hanging out with animals, he's apparently about the same size as they are, and when his toys are out and about, they seem to be life-size. There is one occasion on which Kay explicitly shrinks, in order to go on a voyage in a model ship crewed by mice, but even then there are times in the voyage where Kay seems to be his normal size again and the ship and mice have implicitly grown to match.