I agree with the other readers who say that this is not the best of the Diamond Brothers stories. There isn't much of a plot in this one, nor is there very much action; the jokes and witty remarks are not quite as funny as they are in some of the other stories.
As the story starts, the brothers are broke and Christmas is not far away. Their affairs improve when Tim is retained to protect a glamorous international celebrity called Minerva, a pop princess and film star of Greek origin, who is receiving death threats on the occasion of her visit to London to switch on the Christmas lights and open the Santa Claus grotto in Harrods. Her husband insists on hiring a private detective, both for extra security and to find the originator of the threats, and her manager is ordered to find one and bring him in for approval. We need to wait until the end of the story to learn why the obviously idiotic and incompetent Tim was selected for the job. I hadn't realised that Tim was 28 years old by the way. After making a fool of himself yet again, he actually makes some sensible remarks about his unsuitability for the private detection business. As usual, it is Nick who notices anomalies, solves all of the mysteries and identifies the would-be killer and the reasons for the threats. The story that began on such a low note ends on a much higher one when the manager gives the brothers a £10,000 cheque in gratitude for all they have done to further the career of Minerva, his most famous and lucrative client.
Minerva does seem an unlikely name for someone of Greek origin: I realise that it is supposed to make us think of Madonna, but Minerva was the Roman name for the Greek goddess Athena. This is just a minor objection, and although it is not the best Diamond Brothers adventure the story does have some good features. The scene where the brothers rush to tidy up their messy office when there is an unexpected knock on their door is very amusing. I also particularly liked the references to Regent Street: it is so true that it is full of shops selling clothes that we couldn't possibly afford and wouldn't want to buy even if we could, and that the Christmas lights are no longer worth travelling to see. Obvious, but spot on. Anthony Horowitz often comes up with observations like this, which is why I continue to read these stories.