Strong Poison Part III contains spoilers

Peter has only a month before the new law sessions when a new trial will happen... So he and his organisation have to work fast.  He and his friend Marjorie Phelps, an artist, conduct  researches in Bohemian London, to find out more about Philip Boyes.  He was the son of a clergyman with many respectable middle class relatives, such as his cousin Norman Urquhart... a solicitor who had offered him a home after he split up with Harriet.. but who was not willing to help him financially.  Peter attends a party in Bohemia where he talks to some of Philip's friends. 

 Peter manages to get one of the women who works for him into a post as legal clerk in Urquhart’s office.    He discovers that Philip had an elderly great aunt, a former actress who was very rich. And that she’s living in the country, a helpless invalid.  He sends Miss Climpson, his chief detective, to her home town to try and find a way of getting into her house.

In spite of the serious nature of the case, the book has a lot of comic bits and Miss Climpson’s befriending of the old lady’s nurse is very amusing.  She finds out that the woman believes in Spiritualism and uses this to find the woman’s will. 

 Miss Climpson is voluble and very religious but she quite enjoys using her wits to find out things, using deception and guile. She has to pretend to be a medium, and use an Ouija board.  Joan Murcheson, another of the detective ladies, uses her job in Urquhart’s office to find out more about him... and they realise that Norman Urquhart had a big motive to kill Phillip Boyes…

She finds Mrs Wrayburn's will in her house, and the nurse sends it to Norman Urquhart. Miss Climpson manages to take a quick look at the Will and finds that Mrs Wrayburn left a substantial sum to Philip Boyes, and the rest to Urquhart. However, it appears that Urquhart had been involved in playing the stock market to try to make money and he was using Mrs Wrayburn's money and he lost a lot of it.

The snag is that it seems that Norman shared Philip's last meal.. and ate the same food…. Peter gets Joan Murcheson to get into Mr Urquahart's office at night, and she finds a packet of white powder which when tested proves to be arsenic. Peter works out that Urquhart has lost most of Mrs Wrayburn's money through his unwise speculations, and if Mrs Wrayburn dies, and she is very old and ill, he will have to account to Philip, who has inherited most of the money, for what he has done when acting as Mrs Wrayburn's trustee. So he makes up his mind he has to kill Philip. Peter takes some time to think and read up on arsenic poisoning, to work out how it was done. Bunter has been going out with Hannah Westlock, Urquaharts maid, and learns that the meal was shared between the 2 cousins and that it was hedged about with precautions. Everything that was eaten was eaten by both of them. He learns however that its possible to build up a tolerance to aresnic by eating a little every day, and works out that Urquhart must have done this and so was able to eat an omelette which had been made with poisoned eggs and it caused Philip's death, but his cousin was all right. Peter puts forward this theory to Urquhart and startles him into a confession.. He tries to escape but Parker is in the flat and stops him... and charges are brought against him. Harriet is exonerated completely, but she finds that Peter, as soon as the verdict was given, has left the court and driven away. He does not want to pester her into marrying him by reminding her that he has saved her life.