There is absolutely no way to discuss this movie without revealing some aspects of it. On the other hand, this is not a movie that relies on the ending, but one illuminated by it. Like "Sixth Sense", this is a movie that means more on the second viewing.
So, I will give away part of the basic structure of the movie. If you already plan on seeing the movie, there is no reason to continue. If not, you might as well read ahead; it might change your mind.
SPOILER WARNING! For us the viewers, the story starts in the middle. Zach, son of a the famous author of the children's book "Neverwas" quits a position as a psychiatrist at a prominent college to go to a nobody's-heard-of-it institution in the community where he grew up. Zach (we learn quickly) is tormented by the suicide of this father. Like most suicide relatives, he both blames his father and himself. He has divorced himself from the fantastical world of his father's book, from all fantasy at all, from all remuneration from his father's highly successful book. For Zachary, reality is survival.
He meets a delusional paranoid schizophrenic named Gabriel. What we don't get told about Gabriel until the end of the movie is his nightmarish existence as a little boy: being locked up, abused. Gabriel survived this by creating a world of his own, Neverwas. Neverwas is a world of hope and peace, a world inhabited by fairies and in which Gabriel is the benign king.
Gabriel and Zach's father meet in the mental institution. Gabriel is there for his delusions, Zach's father for his bipolar-ism. The father and Gabriel become friends. Zach's father offers his belief in Neverwas. In fact, he takes Gabriel's world and turns it into his story. As each go in and out of institutions, they maintain a correspond of affection and support.
Gabriel's Neverwas is on land that ultimately Zach's father purchases for him. Unfortunately, the father is not able to care for Gabriel or provide him long term security. His depressions win out and he commits suicide.
The conflict/question the movie initially presents - right up until the final revelations - is what is real. The viewer is led to believe there might actually be a Neverwas. This is necessary because we need to see the world from Gabriel's eyes; and to do this we must accept him with condescension. Were we to simply see him as schizophrenic, we might feel sympathy for him, but we would never empathize with him or truly understand his needs.
Unfortunately, this will lead many viewers to think this is another fantasy come true; and they will be disappointed by the "truth." However, the truths that do come out are beautiful and moving; and there is certainly the fantasy of a "happy ending", more than one has a right to expect from reality.
The true story here is how people change: How Zach comes to see the need for fantasy, to forgive his father and himself; how Gabriel out of desperation has his one moment of cold reality in which he can articulate his need for Neverwas.
And the movie has its moments of humor and insight and romance.
For anyone willing to think and be moved, I recommend this movie highly.