Ace Combat Zero features a story mode where players take control of an Ustio mercenary pilot, call-sign "Demon Lord", participating in the Belkan War. Missions range from escort, assault, and defense tasks to airborne missions. The gameplay is fast-paced and focused on aerial combat, rather than realistic simulation.
Players have access to 30 licensed fighter jets and a unique "Ace Style" meter that affects their ranking as they progress through the game. This system allows players to choose between being a "knight", taking out intended targets with minimal civilian casualties; a "soldier", targeting military objectives while minimizing harm; or a "mercenary", engaging in destructive tactics. As the player's style evolves, the game adapts, sending new and formidable squadrons to challenge them.