When high schooler Kosukegawa Teruharu spots a girl from his school, Gettou Motoko, being hit on by a delinquent, he musters his courage to step in and save her, but is shocked when she suddenly sends the offender flying on her own! As he begins to become closer with Gettou, the two find out that she has DID - when Motoko is in danger, one of three alters comes to the front, each formed to handle the brutal training received from one of their three adoptive fathers. Hibiki holds their training from a legendary karate master, Fujiko their training from a master of weapons and tactics, and Mikiri their training from a grandmaster in an ancient branch of jujitsu. Despite this complication, Kosukegawa becomes determined to love and support them as their boyfriend, and along with the four becomes embroiled in various conspiracies and accidents.
How is it plural?
While she herself does not know the term at first, Gettou is explicitly stated to have DID. Motoko serves as the host, who is initially only partially aware of the others and lacks the ability to share memories with them, and Hibiki, Fujiko, and Mikiri collectively present as a subsystem (referred to as Hifumi) which acts as a protector. Over the course of the series, the four of them gradually begin to understand each other better and work more effectively as a team.
— Major spoiler warning! —
Gettou also has another dormant alter, Zero, who holds the system's anger, as well as the trauma of watching the accident that killed their mother and blaming themselves for it happening. When one of their fathers sends assassins after them in an attempt to draw out Zero, she nearly kills multiple people including Kosukegawa until the other alters are able to intervene and restrain her. From there, the story begins to revolve around attempting to force Zero and Hifumi to fuse in order to ensure Zero can't go berserk, and eventually ends with final fusion between all five.