Miss Cooke is described as being "fair and sturdily built", and her face seems "faintly familiar" to Miss Marple, when she meets her on the tour.
Miss Marple feels that Miss Barrow and Miss Cooke are avoiding her, as they seem anxious to move away if she approaches them.
Miss Cooke and Miss Barrow tell Miss Marple that they take tours together every year, including an Hellenic cruise the year before, a tour of bulbs in Holland the year before, and a trip to Northern Ireland the year before that.
Miss Marple later realises that she had met Miss Cooke in St Mary Mead, before the tour. At that time, Miss Cooke had given her name as Miss Bartlett, and had said that she was staying with a Mrs Hastings, as a companion-gardener. Her hair had been dark, almost black, but she dyed it bright yellow before coming on the tour.
When Miss Marple asks Miss Cooke about meeting her in St Mary Mead shortly before the tour, she thinks that Miss Cooke had been about to disclaim her visit to St Mary Mead, and had looked at Miss Barrow as if seeking instruction on what to say.
Miss Marple engages Miss Cooke and Miss Barrow in a "gardening conversation of some technicality", and Miss Cooke responds. From this, Miss Marple finds that Miss Cooke is not knowledgeable about gardening, as she had claimed to be in St Mary Mead.
After the memorial service for Elizabeth Temple, some members of the tour group decide to continue with the tour, while others choose to stay in Jocelyn St Mary or return to London. Miss Cooke and Miss Barrow change their minds at the last minute, and decide to stay in Jocelyn St Mary. They invite Miss Marple to visit St Martins in the Grove with them.
Miss Barrow and Miss Cooke later come to The Old Manor House, looking for Miss Marple. Clotilde Bradbury-Scott invites them to come after dinner and have coffee.
Miss Barrow and Miss Cooke have coffee with Miss Marple and the three sisters at The Old Manor House. Miss Cooke advises Miss Marple not to drink the coffee, which Miss Marple takes as a warning. Miss Cooke and Miss Barrow then take a "rather fussy departure" in which they come back several times to collect articles they had left behind.
Miss Cooke is later revealed to be sent on the tour by Mr Rafiel to aid and protect Miss Marple, working with Miss Barrow. Miss Cooke had been directed to go to St Mary Mead before the tour, in order to find out what Miss Marple looked like.
Miss Marple explains that after having coffee at The Old Manor House, one of them gave her a warm and affectionate handshake, during which she passed a whistle into her hand. When they came back to retrieve the various articles they had left behind, they probably managed to leave a window unfastened. They then came back into the house while the inhabitants were going up to bed, and concealed themselves, coming out only when Miss Marple blew the whistle. Miss Cooke hid in the wardrobe in Miss Marple's bedroom.
In the BBC 1987 adaptation of the novel, Miss Cooke is portrayed by Jane Booker. Her role and that of her partner Miss Barrow is largely the same as in the original novel. Miss Cooke is seen conducting surveillance on Miss Marple at St. Mary Mead near the beginning of the show but here she does not accidentally engage Miss Marple in conversation nor use the cover story of doing gardening for a village resident. She keeps a distance and watches Miss Marple through binoculars. In this adaptation, the Cooke and Barrow team are also seen shadowing Miss Marple, first during the tour of the gardens at Blenheim Palace and Stourhead and then later on a motorbike at Abbey Ducis. However they are not invited to a meal at The Old Manor House with Miss Marple so they do not have the opportunity to dissuade Miss Marple from drinking a (potentially fatal) cup of coffee or pass a whistle to her. Instead, they manage to catch Miss Marple for a dinner at the Golden Board inn before she goes back to spend the night at the Manor House. Presumably they must have exchanged notes and discussed strategy there because later, when Miss Marple is in danger, they are already in position hidden inside Miss Marple's bedroom to protect her. In this adaptation, Miss Marple does not summon them with a whistle--they could hear what was going on in Miss Marple's bedroom and intervened when they judged the situation was critical.