In the novel N or M?, Betty Sprot first appears as the baby girl of Mrs Sprot staying at the Sans Souci hotel at Leahampton. A playful child, she becomes quite attached to various guests at the hotel including Tuppence.
At the time of the events of the novel, Betty is just over two years old. She does not speak very clearly, but learns to speak better over the course of the novel. She later adopts a habit of "laying her head on one side, fixing her interlocutor with a bewitching smile and murmuring 'Peese.'"
Betty enjoys playing hide-and-seek, and often hides things, including her toy dog, Bonzo.
One morning, Betty enters Tuppence's bedroom early in the morning. She is "both active and voluble", and asks Tuppence to read Goosey Goosey Gander to her. She later removes the laces from Tuppence's shoes, and immerses them in a glass of water.
When Mrs Sprot goes to London for the day, Betty is looked after by various residents of Sans Souci. She asks Tuppence to read to her, but tells Tuppence that the tattered picture books are dirty. She pulls out an identical, clean book from the shelf, for Tuppence to read to her. She then hides the books, and is delighted when Tuppence takes a long time to find them.
Betty goes to town with Miss Minton, to buy a celluloid duck. She is very excited, and chatters about buying the duck to sail in her bath.
While the adults are chatting on the terrace, Betty makes "mysterious expeditions into the bushes", returning to put a laurel leaf or heap of pebbles into the lap of one of the adults. After a while, she does not return to the terrace, but the adults do not notice until Mrs Sprot begins looking for her to put her to bed.
Mrs Sprot discovers that Betty has been kidnapped, and the residents of Sans Souci form a party to go after her.
Betty is taken by Vanda Polonska to a cliff near Ernes Cliff Road. Vanda threatens to throw her off the cliff, and is shot by Mrs Sprot. Betty is unhurt, and runs back to Mrs Sprot.
Betty is later seen coming out of the bedroom occupied by Mr and Mrs Cayley, with a smile of "mischievous and impish glee on her face". It is later revealed that she had been in the room hiding the tattered copy of Goosey Goosey Gander in the bed.
Only towards the end of the story do Tommy and Tuppence discover that Betty is not the real child of Mrs Sprot. She had adopted the child and used her as camouflage for her activities. Mrs Sprot had shot the true mother, a penniless refugee. As Betty is left with no parents at the end of the story, Tommy and Tuppence decide to adopt her.
In Postern of Fate, when Betty Beresford must be in her early thirties, Tuppence mentions that she had gone to East Africa where she "loves poking into African families and writing articles about them." Presumably she had become an anthropologist.