It's eight years earlier, and we finally see what really happened that fateful night, as Chris comes clean to Marie and the others. Two members of the group find themselves in dire trouble while one manages to escape.

A fallen matchstick confirms that the hatch has been opened, validating Marie's letter to Chris. Meanwhile, much to Preston's horror, something has returned for Vivian, speeding up the already ticking clock to find their son.

Lisa is in trouble as Bobby shows his true colors again. The rest of the group takes issue with Serob's frequent disappearances, but soon learns that he's not the killer. Meanwhile, Jessica's grasp on reality is heading down a dark path.

Emma shares her secrets with Jessica. Desperate to find his son, Preston gives a press conference and offers a reward for information. Nora takes Bobby up on his offer, but her heroics end in rage as the others learn the cost of the deal.

Lisa tries to escape from Bobby's grip. Whilst Preston is left to confront his new reality, the Inspectors follow the trail of witnesses. Meanwhile, Chris and the others deal with Jessica, before going after Bobby.

With no doctor to help, another captive falls. Things between Bobby and Lisa continue to approach boiling point. Above ground, the bodies continue to pile up, whilst below, an emotionally unstable Jessica finds an important item.

As the group makes a discovery about the gun, the Inspectors locate Shambala, and call on Preston's influence to get them there quickly. Preston reveals more about that fateful night, and below ground, the mastermind is finally revealed.

With the final revenge for Alex's death in sight, the group learns the last pieces of the puzzle. The guilty are exposed, and sacrifices are made to determine who escapes from Purgatory, and who is left behind to pay for their sins.