With Patient Zero escaped during the first film, The Head Doctor is bleeding to death. A mysterious figure comes into the scene and finishes the Doctor off, taking a orange-glowing stone and scientific research with him. He is revealed as Doctor Porter.
"Mr Blue Sky" Fight Scene
Flash-forward five years later - humanity is on the brink of extinction. It was caused by the outbreak from Patient Zero. We see our three main characters -- Dr. Thomas Brown, Marcus Stone & Sarah Jackson -- returning to their safehouse after an encounter with vampiric zombies.
Sacrifices & Sadness
Thomas, waking up from a nightmare of the orange stone, tries to explain to Sarah that humanity is no more since she believes there is a colony of survivors in the north. In the near-distance, a massive horde of zombies approach the safe house, forcing the trio to board up the windows and lock the doors.
Thomas improvises a defence plan to protect Patient Infinity and the Orange Stone from the oncoming horde, with him and Marcus as the front line and Sarah protecting the Patient. As the pair fight off the horde, we hear "Mr. Blue Sky" by Mr Electric Light Orchestra in the background with the characters' movements matching the music, until the fight ends with a grenade blowing up.
The trio head over to the laboratory, where Patient Infinity has siphoned the energy of the Stone and helped create a 'cure'. Suddenly, the Alpha Male Zombie and another horde arrives, trapping the trio. Thomas realises the cure cannot be destroyed, so they all run to the exit. He then sacrifices himself to let Sarah & Marcus escape. Marcus, bitten earlier in the film, turns into a zombie and is killed by a heartbroken Sarah.
An emotionally-destroyed Sarah is the last one to survive. When she goes outside, she sees more zombies, but they end up being shot off-screen by military soldiers on patrol for survivors. Sarah is saved, and she hands over the cure and the stone to the soldiers, feeling succeeded in Thomas' goal.