It opens in blackness, in confines that lapse memory, to a prison-esque experiment called "The Maze."
Thomas, the gifted prodigy, and newest member of the Glade community finds himself in a strange world where lost boys have created a civilization surrounded by an unsolvable maze. His arrival triggers important changes in the previously routine society.
And that is where the connections end.
The book, true to its dystopic young adult genre, chronicles a world which functions in the hands of Draconian scientists - The Creators. Monitored by Beetle-blades, citizens of the Glade are constantly under the watchful eye of the Creators; a fact the boys know, but only on a surface level. The relationship with the Creators seems symbiotic: once a month a new boy; once a month new supplies; it never rains; the Maze always shuts at night; the Grievers - the Maze's mechanical and modern take on Minautors - mostly never come out during the day.
The book knows where the true villain lies - in the hearts of the Creators, who have subjected teenagers to a series of vicious and deadly experiments with the noble intention of making the recently destroyed Earth a better place. When the going gets bad in the novel, the community - save Gally, a chronic dissenter, and Alby, a recent dissenter - sticks together and perseveres for the greater good and the greater goal: to find a way out of the may. This rectitude and acumen, we later learn, deems this experiment successful.
Instead of focusing on the communal integrity and perspicacity of this young group, the director of the film allows the community to divide when there is a problem, all the runners (except Minho, the Keeper of the Runners) to quit when the going gets tough, and for the Mensa-esque crowd to never actually have to be intelligent. Rather, Minho and Thomas find in the mutilated carcass of a Griever a golden ticket that magically opens a new part of the maze, which the community walks right out of. Furthermore, once the walls of the Maze fail to shut - signaling the beginning of the end - Gally overthrows the once-autonomous collective and tells them, "Good luck against the Grievers." The film's choice to focus on this places the culpability on the Grievers instead of the Creators. In fact, in the book, Minho tells Thomas that the Grievers are mean, but they're not too bright. The Creators, on the other hand, have devised an entire world where Goliath maze walls shift in the middle of the night, people arrive via a box and are zapped of their memory, and small beetles broadcast the Gladers' every move.
While I concede that there are many elements of the novel that would have been difficult to communicate on screen - Thomas and Theresa's telepathic connection, the fact that Thomas can remember certain things, but not other things, the hatred Thomas feels towards the Creators for inflicting this burden on a group of children - many other thematic elements of the novel could have been easily conveyed; namely the Draconian nature of the Creators in contrast with the dauntlessness of the Gladers.
Instead, the movie chose to appeal to, let's just say, a broader audience. Absent of any of the subtleties and, really, the point, the movie was action-packed, and that's about it. My suggestion? Read the book.