Basically, the first half-hour of the movie is a giant info dump where Thomas -- nicknamed "Greenie" -- gets to know the other guys in the camp. There's Alby, the group's appointed leader; Newt (who also played in We're The Millers which was hilarious), who's relegated to exposition duty for most of the film; and Gally, the forceful rule follower.
Here, Thomas learns the ins and outs of the Maze, along with its goofy, unending terminology. This includes the "Runners," which are the unlucky chosen few who run the Maze by day in search of a way out. A dude called Minho leads the Runners, and quickly befriends Thomas. Eventually, Thomas becomes a Runner too, and -- surprise, surprise -- he makes unprecedented discoveries that could lead the boys to freedom. Adding a wrench in their plans, the powers-that-be send the first girl to the Glade, Teresa (Kaya Scodelario), which spurs Thomas to double his efforts.
Now, I don't want to spoil the movie for those of you who haven't seen it yet, but overall this movie was spectatular in my opinion. The visual effects and even the acting (me saying this only because the cast mainly consisted of teens) was fantastic. I strongly recommend this movie to all viewers.