Defcon 5 is a first-person shooter game where you play as an engineer sent to update a defence base on a distant planet. However, upon arrival, the base is invaded by unknown assailants from the air and ground, forcing you to rely on your wits and the defence software to survive.
The main gameplay takes place in a first-person view inside the complex, where you must shuttle between shooting down waves of fighters in turrets outside and scouring the base levels for card keys during downtime. You also need to use the "Virtual Operating System" to repair damage, monitor intruders, and place robot sentries to guard key passages.
The game's objective is to find all access keys and increase the software's efficiency to command turrets remotely or set them to auto-attack with a high degree of success. As defending the base becomes more automated and efficient, you have more time to search for keys, aiming to find the clearance to self-destruct the base, defend your escape shuttle, and make it out alive.
Defcon 5 was released on the Sega Saturn in June 1996.