When their mother Sally remarries, Caspar and Johnny Brent dislike their new stepfather, Jack McIntyre, though their younger sister, Gwinny, is less judgmental.

Douglas and Malcolm discover this after Johnny accidentally makes Gwinny fly, and a race begins between the two groups to find out which chemicals are magic.

Douglas has had to visit the mysterious store the chemistry sets came from to find out the antidote for a chemical which has turned Malcolm small, and they are able to attract his attention.

The other chemicals can do interesting things too, and the misadventures forge a bond among the children, as well as a common front against the Ogre.

they spend a miserable day in each other's place before Douglas detects the substitution and figures out how to switch them back; the experience causes them to understand each other a little better.

When Gwinny sees the cake "gone", and hears the Ogre making noises in his sleep, she assumes her scheme succeeded and is filled with remorse.

The Ogre assumes that much of this is childish nightmares, and talks to her for a while before sending her to bed, promising the girl that he will work on trying to understand the children better.

The two drive home to find Johnny slowly regaining visibility – he is being forced outside into the rain by the others, as water is the antidote to that chemical.

Author Neil Gaiman, when reading the last chapter of this book aloud to his daughter, realised to his amusement that the "classical Greek" spoken by the Hells Angels who sprout in the supermarket parking lot turns out to be colloquial English when the Greek letters are sounded out.