This is a novella in the Rivers of London series.
Abigail Kamara, younger cousin of police constable and apprentice wizard Peter Grant, has been left largely unsupervised while he’s off in the sticks on a case. This leaves Abigail making her own decisions when she notices that kids roughly her age are disappearing–but not staying missing long enough for the police to care.
Natali, a girl she slightly knows, invites her to a “happening” on the Heath. When she goes to the Heath to meet up for the happening, she doesn’t find the girl, but does meet a white boy named Simon. Simon was also invited to the “happening” by a different girl, Jessica, whom he slightly knows. When neither of the girls shows up, eventually they abandon the “happening,” but start to develop an unlikely friendship of their own. Simon tries to teach her to climb trees; Abigail wins a small degree of favor with the housekeeper and Simon’s mother by getting him to actually do his Latin study. They are both studying Latin in the summer, a year or two earlier than expected, because Simon’s mum has ambitions for Simon, and Abigail has (wizardly) ambitions for herself.
When Abigail realizes that Natali and Jessica, the girls who invited her and Simon to the “happening” that never happened, have disappeared, she can’t let go of it. She insists on investigating. Simon introduces her to the Cat Lady, who is a good deal more interesting that that label alone suggests. A talking fox named Indigo approaches Abigail, and says Abigail is needed for something the foxes can’t do on their own.
Abigail, Indigo, and Simon wind up at a very oddly haunted house, and Abigail has to navigate relations with the police, the talking foxes, a river goddess and her court, Peter’s superior officer, Detective Chief Inspector Thomas Nightingale, and, some ways most terrifying of all, Simon’s mum.
Oh, and the haunting at a closed-up house.
Abigail grows, learns, and shows herself as a genuinely kind and decent person. Also a smart and determined person.
Altogether an enjoyable and satisfying story.