Set in the distant future, where a gateway to a warped and chaotic dimensional rift known as "Chaos Field" was opened and the Earth has been invaded by mysterious alien forces. Mankind attempted to put up a resistance against the invaders, but over time, the human race is facing the threat of annihilation and the "Order Field" being consumed by the enemy after many hopeless years with no sign of victory. In a last ditch effort to fight back, three pilots and their dimensional shifting space fighter crafts were dispatched for mankind's final assault against the Chaos Field's corruption.
It is a Boss Game where players face off against a series of bosses across five stages (referred to as "Phases" in-game), each consisting of three boss battles. Players can choose between one of the three pilots and respective crafts, each with their own strengths and weapons: Hal and the "Mixed Blue", Ifumi and the "Flawed Red", and Jinn and the "Fake Yellow". Each craft possesses wing blades capable of sword attacks that can erase some bullets, a Lock-On attack, a Wing-Layer to form defenses against the boss's attacks, and the ability to change between the "Order" and "Chaos" fields, the latter of which will make both the player's attacks more stronger while the boss's bullet patterns become more aggressive.
The game was later re-released for the Wii in 2008 in Japan as part of the MileStone Shooting Collection: Karous Wii (localized in North America as Ultimate Shooting Collection in 2009) with Radirgy and Karous, the MileStone Shooting Collection 2 with the addition of Radirgy Noa and Illvelo in 2010, and the Sakura Flamingo Archives. with the shoot 'em ups from the prior collections with two additional modes for the Xbox 360 in 2014.
Chaos Field features examples of:
- All There in the Manual: The game's plot is told through the manual as there's no dialogue scenes to explain the situation of the story whatsoever during gameplay.
- Auto-Save: The game's home console ports have an Auto-Save option to save high score progress automatically, which is disabled by default in most of the ports save the Wii version in MileStone Shooting Collection 2.
- Boss Game: Chaos Field consists of entirely of boss battles, with each of the game's stages having three bosses with multiple phases. The Original Mode in the PlayStation 2 and GameCube versions has small waves of normal enemies added in between certain Phases.
- Bullet Hell: The bosses in this game can litter the screen with many types of bullets, especially when in the Chaos Field.
- Combos: A combo meter can be built up by canceling bullets with the fighter's Wing-Layer or locking onto destructible bullets with a Lock-On attack while in the Chaos Field.
- Cognizant Limbs: Every boss has multiple parts that can be destroyed, some of which will release Meta Stock/Point Items when destroyed.
- Continuing is Painful: Using a credit after losing all of your shields will wipe your score.
- Crapsack World: The game takes place in being world ruined by the ongoing war caused by the forces of Abo for many years ever since the rift that opened the Chaos Field happened. Many places in the game are barren and desolate in the Order Field, and the dimension appears ruined and destroyed while in the Chaos Field.
- Critical Existence Failure: Player's fighter and the various mechanized opponents the player faces off appear fine until their life meter (or the timer runs out for bosses) reaches zero, in which they explode to pieces.
- Dark World: The titular Chaos Field, where the alien invasion originated from. Compared to the Order Field, the Chaos Field takes a barren, desolate, ruined appearance (e.g. red skies on Earth, destroyed/ruined cities in Phase 1 and 2, the Earth appearing dead in Phase 3 and 4). It also plays a role in the game's central mechanic as player's fighter can shift between the two fields. While Chaos Field, the player's weapon firepower is boosted but so will the enemies.
- Defeat Equals Explosion: Bosses explode upon defeat, with the final boss of each stage explodes in a screen-clearing fashion. The player's ship also explode if they lose all of their shields.
- Difficult, but Awesome: The bosses' bullet patterns become harder and more aggressive and yield less Meta Stock while in the Chaos Field, but your weapons also become even more powerful as a trade-off. Taking advantage of the denser bullet patterns can also lead into higher bullet canceling chains compared to being in the Order Field when at the hands of a player with sufficient skill.
- Difficulty Levels: The standard Easy, Normal, and Hard difficulty levels are featured in this game.
- Essence Drop: Breaking parts from bosses and defeating enemies in the GameCube version's Original Mode will drop Meta Stock or Point Items, depending if the Meta gauge is full or not.
- Every 10,000 Points: Shield extends are earned by scoring 8,000,000 points for the first extend then 20,000,000 points onwards. The home ports offer different extend options, however, the criteria for extends are only told through the game's manual or backing out to the title screen to see the changes.
- Fighter-Launching Sequence: The game's opening cinematic opens with the three fighters preparing to take off from the air carrier above a ruined city below. The sequence continues after the player has chosen their desired fighter, where it flies out to city below before taking off to pursue the first boss.
- Fire, Ice, Lightning: Ifumi's "Flawed Red", Hal's "Mixed Blue", are Jinn's "Fake Yellow", respectively.
- Fragile Speedster: Ifumi's "Flawed Red" has slightly faster mobility than the other crafts, but has a Wing-Layer that only protects her from nearby bullets and a smaller circular Lock-On radius.
- Hitbox Dissonance: Despite how large your fighter's model appears, the hitbox for it is very small and located around the fighter's canopy.
- An Ice Person: Hal's "Mixed Blue" uses ice-elemental weapons.
- Jack of All Stats: Hal's "Mixed Blue" is a balanced craft with average firepower and mobility, a deployable Wing-Layer that can ward off some bullets, and a frontal cone-shaped lock-on sight.
- Life Meter: The Shield gauge, which represents the number of "lives" the player with each unit of Shields.
- Mana Meter: The Meta gauge, which is used for Lock-On attacks and putting up a Wing-Layer for defense.
- Mighty Glacier: Jinn's "Fake Yellow" has slighter stronger damage output than the other crafts and wide range sweeping Lock-on sights, but has slower mobility and a Wing-Layer that randomly scatters flares.
- 1-Up: A few bosses may hold a secret 1-up in the form of the MileStone logo or a cherry blossom in the Sakura Flamingo Archives. version.
- Playing with Fire: Ifumi's "Flawed Red" uses fire-based weapons.
- Roboteching: The Lock-On attacks of the player's chosen fighter and homing lasers from bosses can arch and curve in improbable angles as their home-in on their targets.
- Scoring Points: Points are rewarded for breaking parts of bosses and defeating them. Building up the combo meter can boost the score of the destruction of the bosses and their parts. If a player destroys a part of a boss that drops Meta Stocks when the Meta gauge is full, it will drop Point Items instead that give 5,000 points each. Any leftover time in the timer also provides extra score.
- Score Milking: It's also possible to let cancel many cancelable bullets to raise the combo meter as high as one can possibly raise it before the timer runs out, then start breaking the boss down to get a huge score bonus.
- Shock and Awe: Jinn's "Fake Yellow" uses electrical-based weapons.
- Shoot the Bullet: Lock-On attacks can lock-on to and cancel most enemy bullets while boosting the score multiplier in the Chaos Field.
- Smoking Is Cool: Jinn is seen smoking a cigarette on the character select screen.
- Spread Shot: The spray type for the main shot of Hal's "Mixed Blue".
- Timed Mission: The boss battles in this game are under a time limit, and allowing the timer reach zero will end the boss phase prematurely and score will not be rewarded.
- Two Guys and a Girl: Hal, Jinn, and Ifumi, respectively.
- Updated Re-release: The PlayStation 2 and GameCube versions, subtitled New Order and Expanded respectively, feature a new Original Mode that adds a few enemies and alternate boss patterns, while the GameCube version removes the slowdown that occurred in the arcade and Dreamcast versions.
- Video-Game Lives: As "Shields" with the standard "3 lives per credits" affair being present in this game, although players can increase the maximum number of starting Shields. Up to a maximum of 5 Shields can be carried.
