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Niggle lives in a society that does not much value art. Working only to please himself, he paints a canvas of a great Tree with a forest in the distance. He invests each and every leaf of his tree with obsessive attention to detail, making every leaf uniquely beautiful. Niggle ends up discarding all his other artworks, or tacks them onto the main canvas, which becomes a single vast embodiment of his vision.
However, there are many mundane chores and duties that prevent Niggle from giving his work the attention it deserves, so it remains incomplete and is not fully realized. At the back of his head, Niggle knows that he has a great trip looming, and he must pack and prepare his bags. Also, Niggle's next door neighbor, a gardener named Parish, is the sort of neighbor who always drops by whining about the help he needs with this and that. Moreover, Parish is lame and has a sick wife, and honestly needs help – Niggle, having a good heart, takes time out to help.
And Niggle has other pressing work duties that require his attention. Then Niggle himself catches a chill doing errands for Parish in the rain. Eventually, Niggle is forced to take his trip, and cannot get out of it. He has not prepared, and as a result ends up in a kind of institution, in which he must perform menial labor each day. In time he is paroled from the institution, and he is sent to a place 'for a little gentle treatment'. But he discovers that the new country he is sent to is in fact the country of the Tree and Forest of his great painting, now long abandoned and all but destroyed (except for the one perfect leaf of the title which is placed in the local museum) in the home to which he cannot return – but the Tree here and now in this place is the true realization of his vision, not the flawed and incomplete form of his painting.
Niggle is reunited with his old neighbor, Parish, who now proves his worth as a gardener, and together they make the Tree and Forest even more beautiful. Finally, Niggle journeys farther and deeper into the Forest, and beyond into the great mountains that he only faintly glimpsed in his painting.
Long after both Niggle and Parish have taken their journeys, the lovely field that they built together becomes a place for many travelers to visit before their final voyage into the Mountains, and it earns the name "Niggle's Parish."