The game folows the first Armored Core game in that all the corporations and weapons of the previous games carry over. It is in a sense a direct sequel to the first game.
A mid-sized subterranean complex, large parts of which have been abandoned. Rumors that a research group known as "The Doomsday Organization" is using these empty sections of the city for their own testing and operational facilities.
An organization surrounded by mystery. In addition to the many rumors that surround their Amber Crown activities, they are rumored to the beneficiary of financial backing contributed by one of the conglomeration of companies. They are the masterminds behind the shady "Project Phanstasma."
The codename for an experiment that somehow molds human life and machine together, with the result being a much more efficient AC. However this experiment is so extreme that its side effects can be drastic.
The player is a Raven from Isaac City who receives an unknown transmission to destroy a materials dump in three minutes. Afterwards, the Raven is given his first assignment from the mysterious employer: infiltrate the underground city Amber Crown. Shortly afterwards the Raven is introduced to two Ravens. Stinger, and irregular who is working against the player and his employer, and Sumika, a prisoner of the Doomsday Organization and the one who gave the Raven the assignments. After the rescue operation, the Raven and Sumika work together to take down the Doomsday Organization and their secret weapon "Project Phantasma."
The two partake in acts of sabotage against the Doomsday Organization, and run into Stinger on multiple occasions. Even after destroying the base of operations for Doomsday, the group continues to operate and work on Project Phantasma, with Stinger backing them. The Raven encounters Stinger piloting the prototype Phantasma in the North Highlands, and chases down the final version of the weapon in the subways of Amber Crown. Unfortunately, Stinger escapes with the finished weapon, and challenges the Raven to a dual in the building known as Abyss. The Raven is successful in destroying Project Phantasma along with Stinger, and Sumika is greatful for his assistance. A carrier arrives to pick up the Raven and leave the area, taking the Raven home.
Project Phantasma is a direct sequel to Armored Core, and the core gameplay remains the same. When starting, the player can choose to either start a new game with a starter AC and zero credits, or transfer a save file from Armored Core into Project Phantasma, bringing over all parts and credits acquired. Human PLUS and Overweight modes can be transferred as well if they have been unlocked.
Aside from the mission mode and mech customization, a new mode is introduced to Project Phantasma. The Arena is a mode where the player fights against 49 ranked ACs for credits and extra parts.
All parts from the original Armored Core can be transferred to Project Phantasma, along with over 20 new parts. Some can be found in the shop right at the beginning or are earned by completing matches in the game's Arena mode.
Armored Core: Nexus Disc two features remakes of specific missions from ACPP. Ariake and Vixen's parts are completely unlock-able in Armored Core: Nexus and can be used in the game's conventional single player.