Ace Combat 04: Shattered Skies (released as Ace Combat: Distant Thunder in Europe) is a 2001 combat flight simulation video game developed and published by Namco for the PlayStation 2. It is the fourth entry in the Ace Combat series and the first in the series to be released for the PlayStation 2. The game's plot, set in a fictional alternate universe where Earth has been ravaged by asteroid impacts, follows the player character "Mobius 1", a fighter pilot in a multinational military coalition who spearheads the liberation of the fictional continent of Usea from the expansionist country of Erusea. Ace Combat 04 features arcade-style gameplay with elements of both realism and fantasy; for example, while the game has realistic flight controls and is set in the modern day, the player's aircraft can carry dozens of missiles in hammerspace and conduct difficult flight maneuvers such as flying through tunnels, and superweapons such as massive railguns appear in certain missions. The game features an 18-mission campaign and a multiplayer battle mode. A total of 21 aircraft, including both real and fictional aircraft, can be unlocked and equipped with a variety of special weaponry. Ace Combat 04 began development as a reboot of the franchise, following the lukewarm reception of Ace Combat 3: Electrosphere. The game was developed by "AC04 Project", a team of developers who previously worked on Ace Combat 3. Ace Combat 04 was critically acclaimed at launch, with critics praising its gameplay, controls, plot, graphics, and music. The game was the highest-selling entry in the series until it was succeeded by Ace Combat 7: Skies Unknown in 2019.