After being rescued from the Maze, Thomas and the remaining Gladers are taken to a facility run by Mr. Janson. He explains that the facility is a safe haven, protecting them from WCKD, the Flare virus and the Cranks

Thomas becomes curious about their activities,and he finds out that WCKD's leader, Dr. Ava Paige, is still alive, and that Mr. Janson is working for WCKD, as they discuss about the experiments on the Immunes and that Janson has yet to find the Right Arm, a resistance group located in the mountains. The group escape the facility, as Janson orders his WCKD troops to track them down, where they find themselves in the deserted outside world

After travelling for some time, the group arrive at an abandoned marketplace, where they first encounter the Cranks, humans zombified past the Gone, infected by the Flare

you were walking behind Thomas, when without a warning a Crank ran to you