Decay

A security guard walks the LHC maintenance tunnel and encounters an unidentified man he recognizes. He warns the intruder that he is not supposed to be down there. The man shoots the guard dead, setting off the crisis that follows.

Physicists Connor, James, Amy, and Matt attend an emergency meeting at CERN. Dr Niven argues for stopping the LHC to assess safety, while the Director-General overrules him, insisting the LHC must run because the Higgs boson is essential. A warning is issued about a potential catastrophe if the event recurs.

The Director-General appoints a four-person team to investigate the LHC beam dump. Connor, James, Amy, and Lisa volunteer to join Dr Niven, but Lisa later withdraws to attend a Barcelona conference, so Matt is brought aboard as her replacement.

James introduces his girlfriend Kate to Connor, Amy, and Matt, adding a personal dimension to the crew. The team prepares to investigate the LHC while navigating their relationships.

Amy uses the mainframe to simulate how Higgs boson radiation would affect human tissue. While she runs the simulations, the LHC beam monitor shows the accelerator is active, and James notices the system has powered on by itself, seemingly independent of Amy's actions.

Dr Niven’s team descends into the LHC tunnel as Connor and James attempt to shut down the accelerator. The emergency shutdown switch fails and a radiation alarm sounds. With the elevator not functioning and no mobile signal, they decide to descend further into the maintenance tunnel and James manages a manual shutdown from within.

The team in the tunnel is attacked by Dr Niven's technicians and Kate is captured. They barricade themselves in a room while James uses coolant to deter their pursuers. They discover the interlock array has been sabotaged and find the dead security guard; the technicians show radiation burns yet remain moving, hinting at something horrific.

The group seeks help from the mainframe server room. Amy and Connor breach the firewall while James and Amy analyze the simulations, which reveal that boson radiation could destroy the brain. They realize the technicians are dead and have become zombie-like; a gun is recovered from the fallen guard to defend themselves.

Connor and Amy unlock all doors using the mainframe, but James succumbs to his injuries and becomes a zombie, attacking the group. Matt bolts in panic, and Connor shoots James to stop him. The survivors flee through the maintenance tunnels as the zombie outbreak spreads outside the facility.

Amy escapes to the surface and is chased by the pursuing zombies. She heads to Dr Niven's office to gather data on his simulation to expose the Director-General and tries to call Lisa to report his responsibility. The call drops into Lisa's voicemail, delaying the exposure.

The Director-General storms Dr Niven's office, grabs the gun left by Amy, and confronts her. As he begins to reveal his confession, Amy's call is revealed to be still connected to Lisa's voicemail. The confession is heard only on voicemail as the screen goes black and a gunshot is heard.