Again like The Haunted Woman and Sphinx, The Violet Apple opens with its protagonist receiving an inheritance. Only, in this case, it is not money.
Anthony Kerr is a successful playwright, a sort of cross between George Bernard Shaw and H P Lovecraft: he can present philosophical arguments to the public in an entertaining way, but only by disguising his basic outlook, which is that men are little better than insects in the face of vast, cosmic forces.
While Kerr is polishing off the first act of his new play, a parcel arrives. It contains a family heirloom that predates the Crusades, a glass snake containing a withered pip supposedly taken from the Biblical Tree of the Knowledge of Good and Evil that caused the Fall of Man. When a visiting friend, Jim Lytham, accidentally breaks the ornament, Kerr pockets the seed.
Lytham's visit was to announce his impending marriage to Haidee Croyland. At the ensuing engagement party, Kerr announces that he too is to marry, and his bride is to be Lytham's sister, Grace. Haidee is less than enthusiastic at the news, either through jealousy or because she does not know her own true feelings. She surreptitiously demands that Kerr meet her by an old ruined tower the next day or she will call off her engagement to Lytham.
The meeting starts a string of complications. Lytham and Grace, out for a morning walk, happen upon Haidee and Kerr, and when the two refuse to explain the purpose of their rendezvous, Lytham and Grace both start to have doubts about their respective engagements.
The complications escalate. Haidee seems to be unable to make up her mind whether her true feelings are for Kerr, and whether she should pay any attention to them. They are caught at several other meetings; Lytham stops speaking to Kerr, and there is talk of calling off the marriages.
In the meantime, Kerr has given the seed to Lytham's other sister, Virginia, who has planted it. A tiny, withered tree grows remarkably quickly, and produces two small, violet fruits. When things between Haidee, Kerr, Lytham and Grace come to a head, Haidee, who has an impulsive personality occasionally reminiscent of Krag's, snatches one of the fruits and eats it. She then leaves.
Kerr receives a letter from her shortly afterwards, asking him to eat the remaining fruit and tell her of his sensations. He does so, and enters a state of profound insight. He realises how nakedly his soul is written on his face (a parallel with Adam's realisation, after eating the fruit, that he was naked), and that he can read others' true natures on their faces. His fiancé Grace suddenly appears to him as completely banal and prosaic, and he realises that Haidee is the only person who has any meaning for him.
He goes to see her, and she seems to him to be angelic. But she does not reciprocate his feelings. Although she felt the same towards him shortly after eating the fruit, she points out that she did so thirty hours before he did, and that now, not only have the convictions behind those earlier insights departed, but she has also lost all sense of beauty in the world.
Lytham turns up just as Kerr is on his knees before Haidee, and breaks off his engagement. Haidee persuades Kerr to leave. He does so, and finds that he, too, cools down. Not only does he lose the conviction that Haidee is a goddess-like being, he also loses all interest in his art. He tries to mend things up with Grace but fails.
Some time later, going for a walk he happens upon Haidee, at a place where two trees form a sort of cross — a place he recognises from a landscape painting he bought shortly before the beginning of the novel, and which impressed him at the time as expressing a part of his destiny. At first, the couple are dejected, and seem resigned to the tragedy of the loss of their feelings for one another. Then Haidee says that, although they might never recapture the intensity of feeling given to them by eating the violet apples, they could at least work towards it — "and then it will be ours, not a free gift this time, but ours."