JUST HORROR

The protagonist of The Evil Within is veteran police detective Sebastian Castellanos. He is aided by his colleagues Juli Kidman and Joseph Oda in confronting the invincible apparition Ruvik.

While investigating the scene of a gruesome mass murder at Beacon Mental Hospital, Sebastian, Joseph, and Kidman encounter a malevolent and powerful force. After seeing the slaughter of his fellow officers by a man wearing a white hood and covered in burn scars, Sebastian is ambushed and knocked unconscious. He awakes in a deranged world with Krimson City undergoing massive earthquakes, unreal shifts in terrain and hideous creatures called Haunted wandering the landscape.

He encounters a doctor named Marcelo Jimenez, who is searching for his patient, Leslie Withers. When Sebastian and the doctor find Leslie, they also encounter Ruvik, the man who slaughtered police officers earlier. Ruvik seems to have control over reality through teleportation and changing the terrain; he uses this to separate the trio and trap Sebastian in different horrifying locales that seem to be tied to the memories of individuals that Ruvik has experimented on. Elsewhere, Kidman seems to be pursuing Leslie more fiercely and appears unaffected by Ruvik's control.

Sebastian learns of the story of Ruvik, born Ruben Victoriano, through various flashbacks. Ruben was an intellectually gifted but mentally unstable child, and was close to his sister, Laura. While playing in a barn on their family's property, several men approached and set fire to the building as an act of vengeance against their parents, who had been buying and managing large amounts of land in the area. The fire killed Laura and left Ruben severely disfigured; Ruben's father began hiding him in the basement of the family manor. Deeply traumatized over Laura's death, Ruben eventually killed both of his parents, took control of their fortune, and continued to "donate" money to Beacon Mental Hospital in exchange for test subjects for his experiments.

Ruvik continued his experiments on the human psyche and mind for a time, until Jimenez betrayed him upon the near completion of their primary project, STEM: a machine meant to unite minds into one, felt and experienced by all those linked to it. Jimenez' apparent employer, an unknown organization, subjected Ruvik to a similar ordeal to what he had inflicted on others. The organization apparently abducted many citizens of Krimson City to test the STEM device, essentially linking everyone to Ruvik's world.

When Dr. Jimenez, aided by Sebastian, tries to use Leslie to return to reality, he realizes that they are inside Ruvik's mind, and Ruvik wants to transfer his mind to a capable host and escape into the real world. A beast created from Ruvik's subconscious then assaults the group and kills Jimenez. Sebastian escapes after defeating the beast and reunites with his partners to head for Beacon Mental Hospital, as they and the Haunted seem to be attracted to its lighthouse. Ruvik separates the group once more. Julie finds Leslie first and is about to shoot him when Sebastian and Joseph intervene, prompting her to shoot and kill Joseph before Ruvik separates them again.

Sebastian finds Leslie and guides him back to the hospital where, at the top of the lighthouse, he sees his own body in a tub hooked to Ruvik's brain. Julie arrives and attempts to convince Sebastian that Ruvik intends to use Leslie to cause widespread harm; she implies that she knows more about STEM and the chaotic reality they are in, but Sebastian holds her at gunpoint and is hesitant to believe her. Ruvik interrupts them, gets hold of Leslie, and absorbs him into the brain, throwing the world into even greater chaos. Sebastian battles a gigantic creature created by Ruvik's subconscious and defeats it, waking up in a bathtub. He pulls himself off the machine, removes Ruvik's brain from the STEM, and crushes it. He wakes up again in a tub with Kidman by his side, monitoring him; she motions for Sebastian to stay quiet. Sebastian falls unconscious soon after, and later wakes up in the same tub with no one around except for the other subjects in tubs, including Jimenez in a catatonic state. As police reinforcements arrive, he exits the hospital and notices Leslie walking out of the main gates. Sebastian has a brief headache -- a sign of Ruvik’s influence on subjects within the STEM -- which causes him to lose sight of Leslie. It is implied that Ruvik has returned to the real world through Leslie, and is able to influence those who were connected to his machine in reality, intending to take revenge on the organization that betrayed him.