In the novel 4.50 from Paddington, Lucy Eyelesbarrow is a young friend of Miss Jane Marple. She takes a job at Rutherford Hall over Christmas, to help Miss Marple to investigate a possible murder. She is thirty-two years old.
She has a First in Mathematics from Oxford University but knows that scholars never earned much money. She also likes meeting all sorts of people, and does not want to be with the same people all the time. She took advantage of the shortage of skilled domestic labour, and entered the field, to the amazement of her friends and fellow-scholars.
By the time of the events of the novel, Lucy has become known all over the British Isles. She is "unbelievably competent in every conceivable sphere". She is also willing to do any kind of work, from scrubbing the kitchen floor, to carrying coals.
Lucy never accepts an engagement for a long period of time, and her usual period is a fortnight, or a month at most, under exceptional circumstances. She had been offered enormous sums of money to take a permanent post, but she had no intentions of doing so, and she also does not book herself for more than six months ahead.
Lucy is also in the habit of keeping certain free periods in between engagements, which allow her to either take a short luxurious holiday, or accept a position at short notice if it takes her fancy.
Lucy first met Miss Marple two years before the events of the novel. Miss Marple was recovering from pneumonia, and Raymond West engaged Lucy to look after her.
After Elspeth McGillicuddy tells Miss Marple that she saw a woman being strangled on a train, Miss Marple thinks that the body must have been thrown out of the train. She thinks it is somewhere on the grounds of Rutherford Hall. She engages Lucy to get a post at Rutherford Hall, and look for the body.
Lucy obtains a post at Rutherford Hall, and searches the railway embankment for signs of a body having been thrown from a train. She finds a torn scrap of fur impaled on a thorn bush growing halfway up the embankment, and a powder compact in the grass at the foot of the embankment.
Lucy receives a lot of attention from the men at Rutherford Hall. Luther Crackenthorpe shows her gold coins that he has stored in his room for future use, saying he does not want her to think he is a "played-out sick old man". He also tells her not to throw herself away on a young man. Lucy is not sure whether this is a conditional proposal of marriage. Alfred Crackenthorpe tells her that he has fallen for her, and asks her to marry him. Harold Crackenthorpe says that he has been struck by her ability, and offers her a place in the firm, promising her a good salary with brilliant prospects.
Lucy describes Bryan Eastley as being "like a dog that wants to be taken for a walk", and Miss Marple playfully asks if Lucy takes him for walks. Bryan is frequently in the kitchen with Lucy, and is also described as "watching her with a kind of dog-like attention".
Later in the novel, when the family is taken ill, Lucy calls Dr Quimper. She helps to look after the family while they are recovering.
When Miss Marple is laying a plan to trap the murderer, she engages Lucy's help. Miss Marple and Elspeth McGillicuddy are invited to tea at Rutherford Hall, along with the family and Dr Quimper. Lucy serves fish sandwiches with tea, and Miss Marple pretends to choke on a fishbone. This creates a scenario where Elspeth can see the murderer from behind, holding a woman by the throat, and she is able to recognise the man she saw strangling the woman on the train.
At the end of the novel, Miss Marple says she would not be surprised if Lucy got married soon. Inspector Craddock asks if she knows whether Lucy will choose Cedric or Bryan, and Miss Marple says that she thinks she knows.