In the novel Nemesis, Professor Wanstead is one of the fifteen people on the Famous Houses and Gardens coachtour with Miss Marple. A doctor by training, he specialises in criminal pathology and psychology. He is often consulted by and has done much work for the Home Office and he is widely connected with the authorities such as chief constables, Scotland Yard and prison governors. He knows the Home Secretary well enough to call him by his first name. As Miss Marple would discover towards the late part of the tour, Wanstead is, like herself, on the coach tour by Mr Jason Rafiel's arrangement. After he made himself known to her, they collaborated and he assisted her with the investigation which Rafiel had asked her to carry out.
As Wanstead told Miss Marple, his involvement in the case began much earlier on. A prison governor and a personal friend asked him to examine Michael Rafiel, an inmate of his who had been committed for murder. The governor is not named during Wanstead's conversation with Miss Marple but is highly likely to be Sir Andrew McNeil, the governor of Manstone Prison, as he is present at the meeting with the Home Secretary at the end of the novel. According to Wanstead, the governor was worried. He had good knowledge of the criminal mind and felt that although Michael was a thoroughly bad person and beyond reform, he was, nonetheless, not the type who would kill. The governor was worried about a miscarriage of justice and asked Wanstead for his opinion. Wanstead interviewed Michael several times and conducted physical, mental and psychological tests and came to the same conclusion.
Wanstead approach Jason Rafiel about his son. Rafiel wanted justice but also realised that he expected to die in a matter of weeks. He did not have time to do anything through the regular processes of investigation, through lawyers or government channels. Rafiel thus devised a plan to have Miss Marple invited on a coach tour. Wanstead was to go on the same tour, make himself known to her at a suitable time and render assistance as necessary, possibly even protect her if need be.
Wanstead assisted Miss Marple in various ways. He brought Miss Marple to the hospital to see Elizabeth Temple after she had been fatally injured. Miss Temple had asked to see Miss Marple. It is possible that Wanstead might have impressed on the hospital authorities the importance of granting this. Later, Miss Marple also sent Wanstead back to London. She wanted him to use his connections to trace a parcel which Anthea Bradbury-Scott had posted to a London charity and check what was inside the parcel. Wanstead left for London with some reluctance. Miss Marple wanted to remain at Jocelyn St. Mary to verify certain things at the Old Manor House. Wanstead believed that at this late stage, Miss Marple might be in some danger of her life.
Wanstead met Miss Marple again at the end of the story when he attended the meeting chaired by the Home Secretary where Miss Marple summed up her findings. This meeting eventually led to Michael Rafiel being granted a free pardon. He also brought Michael and introduced him to Miss Marple thereafter.
Professor Wanstead was described as a big man. He had square shoulders and a clumsy-looking body, "looking as though he had been carelessly assembled by an ambitious child out of chunky bricks." Of his face, Miss Marple observed that it "looked as though nature had planned it to be round but the face had rebelled at this and decided to achieve a square effect by developing a powerful jaw." He also had a thick head of greyish hair and "enormous bushy eyebrows which moved up and down to give point to what he was saying." When he spoke his words were like "a series of barks as though he was a talkative sheepdog."
In the BBC 1987 adaptation of the novel, Professor Wanstead is portrayed by John Horsley. His role in this adaptation is largely similar to that in the original novel. He had examined Michael Rafiel, although in this adaptation Michael was not charged or imprisoned. He also assists Miss Marple in the investigation although in different ways. Miss Marple asks him to locate and interview Nancy Brent, the murder of Nora Brent. The tracing of the parcel is done by Broadribb and Schuster instead. There is no summing up meeting at the Home Office. Instead, after Michael Rafiel has been reinstated and has moved back into Rafiel House, Wanstead brings Miss Marple to see him. In this adaptation, Wanstead actually has a Home Office ID card which he showed to a constable, indicating that he is not just a consultant, but likely to be a senior official.