One day after Thomas' arrival, a gi

One day after Thomas' arrival, a girl is delivered through the elevator into the Glade with a note saying "She's the last one. Ever." It implies that there were to be no more children sent into the Glade. The girl subsequently lapses into a coma. When Thomas comes to visit her, he recognizes her, but can't remember her name until he hears her voice telepathically in his head telling him that her name is Teresa.

Teresa wakes up and tells Thomas that they knew each other before they were put in the Glade, and reveals that they can communicate telepathically.

Her arrival triggers everything in the Glade to change: the sun disappears, the deliveries of supplies stop coming, and the doors of the Maze stay open at night, which allows the Grievers to enter the Glade and hunt the children.

Thomas proposes that the walls of the Maze aren't random, but that their movements are actually a code, leading to the discovery that the Maze is spelling out words. Thomas also discovers that the Cliff that they all thought was just a cliff turns out to be where the Grievers leave the Maze or "The Griever Hole". If they can come and go over the edge of the Cliff, then maybe so can the Gladers. This drives Thomas to think they need memories to get out, so he purposefully gets stung by a Griever so he can go through the changing. The changing is a process that begins after one gets the serum, called the grief serum, to cure the infection from the Grievers.

Most of the Gladers decide to make a run for the exit, knowing that it could be suicide trying to take on the Grievers, but figuring that nothing could be worse than being stuck in the Glade. They succeed, only to find out that they were test subjects in an experiment conducted by WICKED. Gally, one of the Gladers who always knew something was different about Thomas and hated him for it, appears at the laboratory to which the teenagers escape. He hadn't originally escaped with them, and he is acting as though he is being controlled. He moves to throw a knife at Thomas, but Chuck, the youngest Glader and Thomas' friend, jumps in front of it.

After briefly mourning the death of Chuck, all 19 of the boys along with Teresa are "rescued" by a group of people and brought to a safe haven while being told about "the Flare" — an apocalyptic occurrence that killed off half of the world's population.

The epilogue reveals that the "group" who rescued the Gladers may just be another variable in the experiment, and that the Gladers weren't the only group being evaluated.