The third season centers on Grace and Simon, the leaders of a cult of rogue passengers, known as 'The Apex', who vandalize the train and assault its denizens to keep their numbers high, believing that the true purpose of the train is to stay as long as possible as a reward. Their travels with a young girl named Hazel and her gorilla friend, Tuba, make Grace more sympathetic to the train's denizens. After Simon kills Tuba, it is revealed that Hazel herself is one of Amelia's creations, and Grace realizes what she thought she knew about the train is wrong. After fighting off Simon's attempt to usurp control, Grace begins facing up to and mending her mistakes.
