It transpires he was chasing whoever stole a packet of fifteen diamonds worth some l0,000 pounds, newly brought from Amsterdam by Arthur Kempster. A dealer with a business in Hatton Garden, London, Kempster had carelessly left the gems unattended for a short time and the miscreant had popped in via a window, taken them, and scarpered. Kempster's absence from the room was so brief he was able to see and run after the thief, engaging the aid of Constable Murray in the pursuit. Thief and constable outpace Kempster so the latter did not see the murderous assault, and the criminal escapes by stealing Dr Oldfield's bike, subsequently found in a cart shed on the London side of town.
The scene then shifts to Marylebone in London, where Dr Oldfield has purchased a practice. One of his patients is Peter Gannet, who lives at l2 Jacob Street -- a thoroughfare with more of its ration of crime! Gannet shares the studio behind his house with his wife's second cousin, Frederick Boles, a maker of jewelry. Gannet is a potter, and among creations displayed on his bedroom mantelpiece is the titular statuette. This monkey and other works do not impress Dr Oldfield much to say the least, for he describes them as "singularly uncouth and barbaric" and exhibiting "childish crudity of execution". Be that as it may, Gannet's illness defies all the treatments prescribed, and so Dr Oldfield, a former pupil of Thorndyke's, decides to consult his old teacher about the case.
They make a startling discovery, pointing to an attempt to murder Gannet, who is admitted to hospital.
Is the culprit Mrs Letitia Gannet, who does not appear to get along with her husband? Or is it Boles, suspected of being over familiar with Mrs Gannet?
Might it be the Gannets' servant, or perhaps even an unknown outside party?
Whoever it is, with their attempt having been rumbled Gannet is certain there will be no further action in that line and returns home. He and Dr Oldfield become friends and the doctor learns a fair bit about making pottery and even tries a hand at it himself, forming a pot which turns up in unexpected fashion later in the book. Despite disagreements between Gannet and Boles, things jog along more or less as normal until Mrs Gannet returns from a week's holiday to find her husband missing and Boles has disappeared. Then a startling discovery is made and Thorndyke is called in to solve the mystery.
My verdict: Although I guessed whodunnit and why before reaching the closing stages of the book, it was more by intuitive leap rather than Thorndyke's careful step by step building up of a case, so I missed some of the more subtle clues planted along the way. There was perhaps one too many coincidences for my taste, although I got a kick from RAF's nod his The Jacob Street Mystery. There's a fair bit of interest in the explanation of the procedure to be followed in bringing a capital case, while the portion devoted to pottery technique may make readers' eyes glaze, no pun intended, but also forms an important part of the narrative.
All in all, however, I found this one of RAF's less interesting works, and so give it a mark of B-. Other readers will probably enjoy it more.