Diana May, a spinster in her forties, goes to work for Féodor Dimitrius, a reclusive scientist who lives near Geneva with his elderly mother. Dimitrius turns out to be researching the secret of eternal youth, and he wants Diana as his first human test subject.
The Young Diana contains examples of:
- Awful Wedded Life: Diana's parents are constantly arguing. Mrs. May is irritated by her husband's vanity and immaturity, and copes with both her marriage and her disappointment in Diana by eating so much that she's morbidly obese. Mr. May sees his wife as dull and unattractive, and takes every opportunity to flirt with much younger girls.
- …But He Sounds Handsome: Inverted. After Diana is turned into a young woman, a friend of her father asks if she's any relation to his friend's middle-aged daughter Diana. Diana says, "Women over forty who have failed to get married shouldn't live! Don't you agree?"
- Emotionless Girl: The youth serum draws its power from the sun and air, and as a result, once it takes its effect, Diana loses all feeling regarding other human beings. The only thing she still feels connected to is nature.
- Everything Sounds Sexier in French: When the young Diana goes skating, a man flirts with her by calling her "Boule de Neige," which sounds more romantic than "Snowball."
- Faking the Dead: When Diana leaves her parents to go work for Dimitrius, she fakes her death by leaving one set of clothes by the cove and throwing her bathing clothes in at high tide to make it look like she drowned. She knows her parents won't miss her, and she doesn't want anyone from her old life trying to contact her. The only person who knows the truth is her friend Sophy Lansing, who she stays with for a little while before she travels to Switzerland.
- I Will Wait for You: When Diana was a young woman, she was engaged to an officer whose regiment was reassigned to India. He promised he'd send for her as soon as money allowed. Diana waited faithfully for seven years. When the officer finally came into a fortune, he immediately broke off his engagement to Diana so he could marry a younger and prettier woman. By that point Diana was past thirty, too old for most men.
- Old Maid: Diana, as well as Sophy, a happily unmarried thirty-five-year-old suffragette who considers herself lucky not to be trapped in an unhappy marriage like so many of her friends.
- The Speechless: Dimitrius employs a mute servant named Vasho who is the only person, other than Dimitrius and Diana, who has ever been allowed to see the device Dimitrius is using to create his youth potion. Vasho communicates with finger spelling, which most people don't understand, so there's no risk of him sharing Dimitrius's secrets.
- That Man Is Dead: After Diana escapes to Switzerland, she feels this way about the Diana of a few months ago, the lonely spinster who could never stand up to her parents. Later, after the rejuvenating potion changes both her appearance and personality to the point where she's almost unrecognizable, she declares that the middle-aged Diana is dead.
- Why Couldn't You Be Different?: At the beginning of the book, Diana lives with her parents, working as an unpaid maid out of a sense of filial duty. She gets little gratitude from her parents, who doted on her when she was younger but have become ashamed of her now that she's an old maid. One night she overhears her parents talking about her, with her father calling her "in the way" and "superfluous" and wishing she'd been trained to do something that would keep her out of the home so he could pretend he didn't have a daughter. This is what leads Diana to decide to finally leave her parents and go work for Dimitrius.
