Tim is described as a "tall, thin young man, with dark hair and a rather narrow chest". His mouth has "a sweet expression", and he has sad eyes and an indecisive chin. His hands are long and delicate.
Some years before the events of the novel, Tim's health had been threatened by consumption, and "he had never displayed a really robust physique".
Tim is popularly supposed to be a writer, but his friends know not to ask about anything he has written.
At the beginning of the novel, Tim is in Majorca with his mother, and suggests that they go to Egypt. When Mrs Allerton mentions that Egypt is expensive, Tim says that he will pay for the trip using the money earned from "a little flutter on the Stock Exchange".
He is related to Joanna Southwood. Although Tim had never demonstrated a sentimental interest in Joanna, nor she in him, their friendship irritates Mrs Allerton. Together they hatched a plan to replace Linnet Doyle's expensive pearls with fakes.
On the Karnak, Tim occupies a cabin on the starboard side, in between those of Mr Fanthorp and Mrs Allerton.
At the beginning of the cruise, Mrs Allerton tells Tim that she asked Poirot to sit at their table. Tim is very annoyed, and says that Poirot is an "unmitigated little bounder". Mrs Allerton is surprised because Tim is usually easy-going and good-tempered, and this outburst is quite unlike him. He is also very cosmopolitan, so she does not think he has the "ordinary Britisher's dislike--and mistrust--of foreigners".
After the death of Linnet Doyle, and the discovery that her pearls are missing, a search of the cabins is made. In Tim's cabin, there is an exquisite little triptych and a big rosary. Poirot notes that there are no letters from Joanna Southwood, and also notices a tube of Seccotine.
It is revealed that Tim and Joanna were working together in several Society robberies, where an imitation piece of jewellery was substituted for the original. Poirot explains that Tim substituted an imitation string of pearls for Linnet's pearls, and it was the imitation which Miss Van Schuyler took, and which Miss Bowers later returned.
Poirot also suggests that the real pearls are concealed in the rosary in Tim's cabin. The rosary has beads which can unscrew, and inside each is a pearl, which is stuck with Seccotine. Poirot also suggests that the imitation pearls had been sent to Tim by Joanna, concealed within a book, with a square cut out of the pages in the middle.
Poirot tells Tim that Rosalie Otterbourne saw him leave Linnet's cabin on the night of the murder, and go to his own. Rosalie had told Poirot that she had seen no one, but he knew that she had lied.
Tim admits to taking the pearls from Linnet's cabin, but he does not admit anything about Joanna Southwood, saying that how he got the imitation pearls is his own business.
Poirot suggests that Tim could have made the substitution before the night of the murder, and Linnet could have realised it. He might then have killed her, as well as Louise Bourget and Salome Otterbourne, to prevent his crime from being exposed. However, he later explains that he only said this because he wanted Tim to realise that there was a good case against him.
Poirot then tells Tim that he has not yet examined the rosary in his cabin, and it could be that when he does, the pearls will not be there. Also, as Rosalie maintains that she saw no one, there is no case against him. Poirot also gives Tim the imitation string of pearls.
Tim throws the imitation pearls overboard, and tells Rosalie that when he returns the box to Poirot, the real string will be in it.
Rosalie asks Tim why he started stealing, and he says that it was a "much more attractive way of earning a living than just pegging away at a job", and that the attraction was mainly in the risk.
Tim decides to tell his mother about his involvement in the robberies, saying that she will be so relieved that his relations with Joanna were purely of a business nature that she will forgive him everything else.
Tim Allerton is not featured in the 1978 Ustinov adaptation of the novel. The entire pearl theft subplot has been simplified, with Marie Van Schuyler stealing the genuine pearls and later returning them. There is no substitution of fake pearls,
In the 2004 David Suchet adaptation, Tim Allerton is portrayed by Daniel Lapaine. This portrayal is quite fairly to that in the original novel. The only difference is that while Rosalie Otterbourne is interested in him, he turns her down at the end, telling her that she is "barking up the wrong tree".