Deadlight is a 2.5D side-scrolling survival horror platformer set in Seattle in the eighties. The story starts when the protagonist Randall Wayne kills Karla Patterson in the warehouse. She was Stella's sister and part of the small band of survivors who hole up in the building to escape the Shadows, the zombie-like undead that chase after them. Karla was bitten and infected. The other survivors in the complex are Sam Powell, Benjamin F. Parker and Stella Patterson. As they attempt to escape to a safe point, a ladder breaks down and Randall stays behind. He faces the Shadows alone but attempts to reach the safe point as well where he hopes to find back his family. Just 145 days earlier a first patient started spreading the virus that decimated society.
Randall is moved over a horizontal 2D plane over a linear path while the environment is in 3D. He can run, kick down doors, operate items, and jump and grab ledges to climb on platforms. The game focuses on escape mechanisms and puzzles while discouraging combat and direct confrontations. Large parts of the game are spent fending off the undead by using items in the background, such as moving a cabinet to block a door or crushing the undead. He can examine various items of interest along with diary pages that are stored in a separate section called Randall's memories. Other times they have Randall make observations about the environment to trap the Shadows. Items that can interacted with are highlighted by blue gears when he is close. Combat is often optional and discouraged, but there are opportunities to fight with a fire axe and a gun. Fighting drains a blue stamina meter quickly and when it is low the screen pulsates in hues of blue. Running also drains it. When too much time is spent in combat, more Shadows arrive and they overwhelm Randall. Once the undead are down they can be finished off. Initially there are three health points that are restored through first aid kits.
The game uses a graphical style where the screen is not always filled entirely with the backdrop. Often, each location fades in and out when it is accessed and larger areas scroll along in a regular way. In small environments the camera zooms in. Cut-scenes are drawn in a comic book style.