Ace Combat 04

The player takes on the role of Mobius 1, an elite fighter pilot serving in the Independent States Allied Forces' (ISAF) air arm, fighting against the Eruseans who have occupied almost all of the Usea continent after an apocalyptic event in 1999. A massive asteroid, codenamed Ulysses 1994 XF-04, struck Usea, killing 500,000 people and turning many into refugees. Erusea use this as an opportunity to occupy the continent after a prolonged military buildup. They achieve military supremacy via the superweapon Stonehenge, a megastructure battery of railguns that were originally designed to destroy asteroids but have been found to be effective against airplanes as well. This allows Erusea to dominate much of the airspace over the continent giving their military a powerful tactical advantage. Consequently, ISAF has been pushed back to the eastern coast of Usea.

Between the missions the story is told through flashbacks, later revealed to be a long letter to Mobius 1 about the war and living inside the occupation. The letters are written by an unnamed boy whose parents were killed when a fighter jet crashed into their house in San Salvacion. This fighter jet was shot down by Yellow 13, the Erusians' top fighter pilot, and a member of the Elite Yellow Squadron. Following his parents' death the boy is taken in by his uncle, in the city who lives above a tavern frequented by Erusean soldiers, a failing taxi driver due to low fuel rations issued by Erusea. While in the tavern, he befriends Yellow 13 after meeting him and the other Yellow Squadron members. He also learns that the owner of the bar is a member of the local resistance against Erusea and becomes a "spy" for them due to his friendship with Yellow 13.