The game takes place in 1995, 15 years before the events of Ace Combat 5: The Unsung War, revolving around the conflict between the nation of Belka and the Allied Forces; a multinational military alliance with forces from the nations of Osea, Yuktobania, Ustio, and Sapin among others. Several characters from Ace Combat 5 make an appearance in Ace Combat Zero, but they appear in tertiary roles. The player plays as a mercenary pilot, callsign "Cipher", hired by the country of Ustio, which was invaded by Belka.