The twins are three, and prone to asking 'Why Because?'. Belinda (Bel) is home from school, her Trinidadian cousins Ben and Charlie are coming to see London for the first time, and the boys are staying with Daisy and Alex. They go to see the Crystal Palace, with Uncle Tom, and their Nanny Gilpin leaves the lavatories in pursuit of another nanny (it turns out later, the nanny has barely washed hands, but the pursuit is because there's a pram with a lifelike baby doll left near the entrance, and she has assumed the nanny has forgotten her charge - which makes NO sense - why would she not take over care of the child herself, instead of haring after a stranger??).
So anyway, the kids see their nanny chasing after another nanny, and go in pursuit. Their nanny eventually ends up in the lake near the dinosaurs, having been attacked, and almost drowns. The two oldest go in to rescue her, and send the smaller boy for Tom Tring.
Meantime Daisy has gone looking for her Nanny, and discovers a stall door ajar, and someone on the seat - fully dressed - still warm, but with no pulse. Charlie eventually gets Tom's help, and Mrs Tring (who later nurses Nanny for at least one night - not sure she ever got paid?) stays with Daisy, who then goes off to call for help.
Alec is in Bristol for a bit, so Inspector McKinnon from Peckham takes on the case, and Alec is called in once he gets home. His Superintendent is retiring, and he thinks about whether to accept the job throughout the book (you find out at the end that he's been offered it, and apparently, has accepted but will have a week's leave first).
The boys do a marvellous job of tracing their route on a map at Scotland Yard, then heading off to the wax museum (having paid to ride elephants at the zoo the previous day - apparently these ran until 1960!).
Alec and his team are tracing the origins of their body, who turns out to be a wealthy man, with a really nasty streak. He knocked over a nearly completed bust, about to be bronzed, and delivered coal to a writer, then used a manuscript to light the fire. He is said to have had a breach of promise suit laid against him by a Russian Jeweller. Daisy and her Indian best friend Sakari? take Daisy's auqua-marines to be reset, and (at the finale) end up being witnesses when the woman elopes with the goldsmith who escaped with the family during the Russian Revolution (to China, France, and now London).
Daisy also drafts in Philip, who is in town without wife (who is pregnant back in America) and makes him take her to the KitKat night club, where she sees a woman who the victim was said to have an interest in. She turns out to have been a good girl, who lives with her parents, and has gone through several previous dance partners. One of whom was someone the victim introduced to her.
He turns out to be the younger brother of a sickly peer, Lord Ledborough, who has just moved to a nursing home in London. Ricky (Alaric) Rexham-Clarke (with or without an E) had originally been a medical student. He attempts (successfully) to make his brother believe that a hypodermic of potassium Chloride will help his condition. If he'd been reasonably well, and the solution diluted, it might have. Alec arrests him (as he's about to inject his brother, after leaving it VERY late) for practising medicine without a licence, and Nanny Gilpin sends a letter after she gets her memory back while staying at her sister's, so Ernie is sent down to get her statement , with several photos, but Alec is still unconvinced he has enough of a case for the CPS.
It's not mentioned, but that means that if the sickly lord dies, they will be attempting to prosecute a peer, and that means (I'm pretty sure) doing so through the house of Lords.
Daisy's previous flatmate - the one who identified Ricky's family - is pregnant, and will probably end up taking Nanny, when Daisy's twins go off to Montessori pre-school. And Daisy's mother is coming to town - and Daisy has done nothing about the list of jobs she was given by Alec's mother.
It is irritating that any time Daisy contacts Alec with useful information, she gets yelled at, and every time she doesn't, she gets yelled at.