The Conspiracy is a group of corrupt and evil adults that secretly controls Tokyo through the use of the Palace for their twisted desires. The Phantom Thieves of Hearts fight against their corruption by stealing their Treasure in order to reform them. Once the Phantom Thieves grew famous the Conspiracy decides to hunt them down and kill them. Their ultimate goal is to help Masayoshi Shido be elected to Prime Minister and implement his self-centered political beliefs, which would give them legal authority to remove any opposition to their desires.
Members who know too much and pose a danger are executed by the assassin, who murders Shujin Academy's Principal Kobayakawa, the Director of Special Investigations Unit, and Kunikazu Okumura when they no longer serve a purpose as members.
While the Principal became an enemy of the thieves after their first heist created a cloud of scandal over the school, the group itself becomes hostile after Junya Kaneshiro is arrested, as he was a primary financial backer of the group and his confession deeply hurt the group's bottom line and resource base. Initially, a tech firm CEO poses as the online hacker activist group Medjed to threaten the Phantom Thieves into compliance (although the ultimate plan was to lose anyway and make them ideal scapegoats via fame), but after Futaba Sakura successfully breaks into his server and defangs him, they decide to try a subtler approach, kicking a heist request on Kunikazu Okumura to the top of the list artificially, as they had planned to assassinate him; after the Thieves steal his Treasure, Goro Akechi kills Okumura's Shadow Self during his confession, causing the public to believe the Thieves had killed him and creating the legal reason to launch a full investigation of them.
Lesser members appear as Cognitive existence in Shido's Palace, but Shido's personality is so strong that many of them turn out to be actual Shadow Selves who did not form Palaces of their own. After Shido is defeated, the Conspiracy is greatly weakened and rendered impotent politically, but the surviving and unredeemed members retain Goro's knowledge of the Palace and are planning use it for even more sinister plans like ensuring both the control of Japan and better foreign relations. With the aid of Yaldabaoth, they also manipulate the media to believe that Shido is a hero and the Phantom Thieves villains. In order to defeat the remaining members of the Conspiracy and reform the whole world, the Phantom Thieves stop Yaldabaoth and dissolve Mementos, preventing anyone from ever taking advantage of the Palace and dissolving the Conspiracy once and for all.
- Yaldabaoth: Conductor
- Masayoshi Shido: Leader
- Goro Akechi: Assassin
- Kunikazu Okumura
- Junya Kaneshiro
- Principal Kobayakawa
- SIU Director