Chasm: The Rift is a 3D first-person shooter. As a Marine, the player has been sent inside a power plant to investigate the loss of power at the plant. They later discover that a group of monsters known as the Timestrikers are using the power to create ruptures in time that allow them to invade the Earth in three different time periods: present day, ancient Egypt, and medieval times. Timestrikers that have invaded the present day are also taking control of a nuclear plant in order to detonate a nuke that will destroy nearly all life on Earth.
The game starts in the present day, as the player goes inside the power plant and nuclear plant, battling strange-looking security guards and security systems that have gone haywire. They will then find the rifts that the Timestrikers are using to invade other time periods, eventually winding up at the Timestrikers' military base in the distant future.