In the novel Postern of Fate, Henry Bodlicott is the grandson of Isaac Bodlicott. He is the second son of Mrs Stephen Bodlicott who is the daughter-in-law of Isaac. After the death of Isaac, Tuppence Beresford asks Mrs Bodlicott if she knows anyone who can help her in the garden. Mrs Bodlicott replies that she would send over Henry to see what he can do. His friends call him “Chuck”.
Henry knows something about Tuppence's background in the N or M? case and is excited to think that something similar is going on in his town. He offers his help and the help of his friends, because they "hear things". Tuppence forms them into an informal "Junior Brigade" to help with her investigations first into the Mary Jordan mystery and into the killing of Isaac.
There is some confusion about Henry's kinships in the text. Henry tells Tuppence that the Isaac who was killed was his uncle. In fact there is a whole chapter entitled "Reminiscences about an Uncle" where Henry talks to Tuppence about him. Yet the ages don't make sense in this case--Isaac is into his nineties at least and Henry is twelve to fourteen. As Henry is Mrs Bodlicott's second son and she is Isaac's daughter-in-law, Isaac must be Henry's grandfather.