The latest mystery featuring everyone's favorite 11 year old girl with the encylopedic knowledge of poisons. Near the beginning, she counts on her fingers and realizes that the next dead body she finds will give her a full hand. She just didn't realize it would happen that soon.
Finances have become dire for the de Luces. After years of fighting Inland Revenue, the tax bill has reached the point where a "For Sale" sign has been posted for Buckshaw. Flavia is facing the loss of her home, in particular her beloved chemistry lab/bedroom. Father, as usual, does nothing other than to retreat into his study and his stamp collection. For the first time Flavia is furious at his passiveness, but struggles to actually confront him (from a previous story we know he owns a rare 1st edition folio that could be sold to solve the tax problem but he hangs onto it because the sentimental value from his late wife is more important to him).
She has been hanging around St. Tancred's Church a lot lately because the organist has disappeared and her older sister Feely is practicing to take his place (the de Luces are Catholic and St. Tancred's is Church of England but father believes in supporting your local industry). Since father doesn't want Feely walking by herself at night Flavia is deputized to accompany her. Feely notices a problem with the sound so she opens the door to the internal workings of the organ and they discover a bat. How did it get into a closed environment and is it the only thing responsible for the organ being off-pitch?
From hanging around the church Flavia learns that scientists are coming to exhume the body of St. Tancred on the 500th anniversary of his death. Even though they are trying to do it quietly it is impossible to keep Flavia away from something like this. Especially when she's told that sometimes the bodies of saints don't decompose she knows she's got to see. The vicar lets her remain after she sneaks in. When the masons manage to create a small hole in the stonework, she volunteers to poke her head through to look. However, instead of the body of a saint, she sees the body of the missing organist, with a WW II gas mask on his face covering what appears to be a lace handkerchief.
As usual, Flavia rises to the challenge (i.e. starts snooping) and spends a lot of time crawling around the innards of the church. It is her knowledge of chemistry that proves key in explaining the bizarre circumstances of the organist's death.
This time there is more back-fill on the characters. Feely may be getting married, although she's trying to keep the name of the suitor away from her sister. Flavia meets someone who had a clandestine relationship with her mother that only Mrs. Mullet knew about. A local busybody and a private detective also provide information about her parents and Dogger. She also gets closer to Inspector Hewitt's wife Antigone, who she had been afraid to go near. She had inadvertently driven her to tears in an earlier book by asking why they didn't have children when they could afford them on his salary? It turns out every child they had died in infancy and Flavia has been avoiding her ever since. However, the biggest shock comes from the vicar.
Flavia has never liked the vicar's wife, particularly after she spanked Flavia for standing on the altar when she was trying to collect a specimen. Even though she learns she was a school friend of her mother, Flavia initially finds it amusing that she scares the #%&* out of her twice around St. Tancred's. Then the vicar explains about a horrific tragedy from their past, something that everyone in Bishop's Lacey knows about but never discusses. Flavia has now accidently reminded his wife of it. Instead of finding it amusing to scare her, Flavia is now mortified that she has caused her such pain and vows to be nicer to her.
There is the usual snarkiness about Mrs. Mullet's lack of cooking prowess (faced with her porridge, Flavia imagines herself a reverse Oliver Twist: "Please sir I don't want any more!") and reminders that her behavior isn't always angelic (she reminisces about getting dishonorably discharged from the Girl Guides for a hide-and-seek prank and being surprised how long it took the victim to stop screaming and foaming at the mouth). Dogger is always there as a father figure to give her the guidance she can't get from her real dad; she can't imagine losing him if they lose the house.
Flavia succeeds in putting together all the clues, although she comes agonizingly close to becoming the murderer's 2nd victim. At the end father drops a bombshell which should segue nicely into the next book. Unfortunately, it will probably be the better part of a year before it comes out.