The plot concerns the activities of a group of church musicians, the Mellstock parish choir, one of whom, Dick Dewy, becomes romantically entangled with a comely new school mistress, Fancy Day. The novel opens with the fiddlers and singers of the choirâincluding Dick, his father Reuben Dewy, and grandfather William Dewyâmaking the rounds in Mellstock village on Christmas Eve. When the little band plays at the schoolhouse, young Dick falls for Fancy at first sight. Dick, smitten, seeks to insinuate himself into her life and affections, but Fancy's beauty has gained her other suitors, including a rich farmer and the new vicar at the parish church.
The vicar, Mr. Maybold, informs the choir that he intends Fancy, an accomplished organ player, to replace their traditional musical accompaniment to Sunday services. The tranter and the rest of the band visit the vicar's home to negotiate, but reluctantly give way to the more modern organ. Meanwhile, Dick seems to win Fancy's heart, and she discovers an effective strategem to overcome her father's objection to the potential marriage. After the two are engaged secretly, however, vicar Maybold impetuously asks Fancy to marry him and lead a life of relative affluence; racked by guilt and temptation, she accepts. The next day, however, at a chance meeting with the as-yet-unaware Dick, surprised Maybold learns from him of his engagement to Fancy. The vicar follows by prompting her by letter, while expressing being taken aback by such news, to be honest to Dewy and withdraw her commitment to him if she indeed intended to become married to Maybold. Fancy responds by withdrawing her consent to marry Maybold and asking him to keep her initial acceptance of his proposal forever a secret. Maybold replies by urging her again to be honest with Dick and admit she accepted the vicar despite having already committed herself to the young tranter, assuring her she would be forgiven. However, as she marries Dewy who is so in love he readily dismisses what he previously (rightly) considered exhibits of her fickleness and rejoices at what he perceives at the prospect of a happy union based on honesty, given Fancy's effusive and seemingly frank admission to some (minor) infidelities, while he assumes they would never keep any secrets from each other, she resolves never to disclose the truly incontrovertible and damning evidence against her character in her having so readily accepted Maybold despite her engagement to Dewy.
The novel ends with a humorous portrait of Reuben, William, Mr. Day, and the rest of the Mellstock rustics as they celebrate the couple's wedding day. The mood is joyful, but at the end of the final chapter, the reader is reminded that Fancy has married with "a secret she would never tell" (her final flirtation and brief engagement to the vicar). While Under the Greenwood Tree is often seen as Hardy's gentlest and most pastoral novel, this final touch introduces a faint note of melancholy to the conclusion.

Question: What secret does Fancy keep from Dick?
Answer: She keeps the secret of accepting Mr. Maybold's offer to marry.
Alternative answer: That she agreed to marry Vicar Maybold