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In the novel Taken at the Flood, Sergeant Graves is the assistant of Superintendent Spence in Warmsley Vale. He is described as a tactful young man with a very good French accent, presumably of good education and general knowledge. He is respectful to Poirot because his superior is, but secretly thinks that the Detective is "one of those frightful old dug-outs".

"'Think you're the oracle at Delphi,' growled the Superintendent. 'Not that I know what the oracle at Delphi was – sort of thing young Graves gives himself airs about knowing – doesn't help his police work any.'"

(Spence to Poirot in Taken at the Flood, Book II, Chapter Eleven)

Spoiler warning: A spoiler is announced! The following section contains details about the plot of Taken at the Flood.

After the death of Enoch Arden, Graves shows Rowley Cloade into Spence's office at the police station. After Spence hears Rowley's testimony, he tasks Graves with the identification of a gold lighter with the initials D.H. on it, which was found in Arden's room. They also discuss a second piece of evidence, a watch with a broken mainspring that stopped at ten minutes past nine. The Superitendent concludes that "it's a well-known hoary old trick" which cannot be relied upon when establishing the time of murder. Graves remarks that the business seems like a clear case, as the lighter clearly belongs to David Hunter who was spotted in the neighbourhood in the evening of the murder. Spence is not so sure. He indicates a third piece of evidence, a red lipstick that was found under the chest of drawers and represents "the unknown quantity" – as there has been no other suggestion of a woman being implicated.

Presumably, Graves is present at the police inquest where Beatrice Lippincott, Major Porter, David Hunter and others testify.

After Major Porter commits suicide in his London flat, the local police telephone to Warmsley Vale and Graves is sent up by Spence to investigate the matter. Poirot, who wanted to call on the Major, is taken up to examine the scene. Graves says that Porter shot himself a couple of hours ago but nobody heard the shot. Curiously, he did not leave a note, which "one would have expected an ex-Army man to do". Poirot agrees. On the stairs, they meet the landlady, and Sergeant Graves neatly detaches himself from Poirot before he can also fall victim to her ghoulish enjoyment.

Spoilers end here.