Caspar, Johnny, and Gwinny's mother has remarried, and they don't get on with their new stepfather, who they privately refer to as "the Ogre", or his two sons, Malcolm and Douglas. As a peace offering, the Ogre buys them a chemistry set, which turns out to be magical. Hilarity Ensues.
Tropes include:
- Adults Are Useless: Both parents remain woefully ignorant of the magic going on around them for most of the book (the Ogre even prevents two of the children from finding the antidote to one of the chemicals at one point), but find out at the end, whereupon the Ogre proves to be very useful.
- Anti-Villain: The Ogre isn't necessarily a bad guy, but he is a grump. He gets better after talking with Gwinny and having a Jerkass Realization.
- Artistic License – Economics: Averted. When the family uses the Philosopher's Stone flakes to turn random items in the house into gold, they only have a small amount and use it on only the ugliest items they don't want. While they sell for a high price at the auction, they don't cause any economic disruption precisely because of the objects' scarcity. When Sally returns, she points out several other things they could use.
- Ax-Crazy: Johnny when he turns invisible ( apparently, turning invisible while already in a bad frame of mind can cause one to go a bit loopy) and Gwinny, to a lesser extent. She baked the Ogre a foul cake mixed with a little bit of everything poisonous after he hit Johnny and Malcolm. It turned out all right though.
- Beware the Nice Ones: Gwinny, whom everyone considers a sweet little girl, and who's the most open to her new family... is also the first to opt for murder as a solution to the situation. Realising that he's driven the peaceable Gwinny to this leads the Ogre to reconsider his behaviour.
- Corporal Punishment: The Ogre spanks Johnny and Malcolm with a backbrush.
- Designated Villain: The Ogre is treated as a villain by the main characters, even when he tries to be nice to them.
- Gold Makes Everything Shiny: Lampshaded when the children start transmuting the ugliest household rubbish into gold — a hideous cow-shaped jug is hideous, but a golden cow-shaped jug is a curio and therefore tremendously valuable to the eccentrically wealthy.
- Jerkass Realization: The Ogre gets this after Sally leaves and Gwinny confesses that she baked a poisonous cake for him. Fortunately he didn't eat it because she unknowingly included a chemical that turned it invisible. This allows the family to mend.
- The Little Shop That Wasn't There Yesterday: The shop they buy the chemistry sets from, which, though it doesn't appear or disappear inexplicably, is demolished at the end (once its work is done).
- Shout-Out: In a scene involving a collection of ugly household objects, one of them is a horrendous cow-shaped jug.
- Snobs Vs Slobs: A bit of this at the beginning. Malcolm is snarky about how messy Casper and Johnny keep his former bedroom, and their response is to mock his accent, which is posher than theirs.
- Wicked Stepmother: Played with: the Ogre appears to be an evil stepfather, but turns out to be fairly sympathetic, if grouchy. Also with quite a dry sense of humour, Casper realises that they have probably been taking his jokes as serious statements.
