Ninomiya is an ordinary high school student who lives at home with his older sister Ryoko. However, Ninomiya's life ceases to be normal the moment a cute girl named Mayu descends into the middle of his school's campus in a military-grade helicopter. It turns out Ryoko sent this girl, along with her older brother Mikihiro, to live at the Ninomiya household.
Episode 1
Shungo meets Mayu at his school, and learns about Mayu's condition.
Episode 2
Reika is introduced and gets worked up when Mayu unknowingly makes fun of her. Reika challenges Mayu to a contest for Shungo, only to lose and be blackmailed into becoming his maid.
Episode 3
Hinako takes Mayu out to get to know her better, while Reika attempts to get rid of Mayu by exploiting her fear of men.
Episode 4
Mayu attempts to help Reika with the housework, only to find a hidden passage instead. They get trapped within the passages for most of the day, only to be abducted by Ryōko for slacking off when they finally escape.
Episode 5
The crew go to the beach for a 'vacation', where Mayu starts getting closer to Shungo as Reika continues to try to upstage her. After witnessing Shungo inviting Mayu to watch the stars together, Reika wanders off in a depression. Shungo runs off to find her, only to meet her inner self instead. Things end with the appearance of a mysterious group of soldiers.
Episode 6
Shungo, Mayu and Reika are attacked by soldiers and are held captive in the house. Shungo fights back but is beaten up after being knocked out by a mysterious woman in a Tiki mask. Reika exploits Mayu's bad luck to trigger the various traps in the forest, spreading chaos among the enemy. She uses this as a diversion to free Shungo, who then faces off against the mysterious woman.
Episode 7
Shungo falls ill after 'losing his fighting spirit' and Mayu tries to nurse him back to health, with disastrous results.
Episode 8
Ryōko orders Shungo to make some ramen for her. He and Mayu spend hours trying to create the perfect ramen, only to realize that Ryōko will settle for a simple cup ramen.
Episode 9
The class is at Kyoto on a field trip, the boys try to get into the girl's room but are hunted down by the various faculty members. After being taunted by Tasuku, Shungo tries to force his way in to see Mayu, only to be intercepted by Irori.
Episode 10
Shungo, Mayu and Reika spend the last day in Kyoto having fun together, like they did back when they were children. Tasuku tries to break them up by using his family's ninja-clan to eliminate Mayu, only to be stopped by Irori. Irori confesses her love to Tasuku and tells him she has only loved him, not Shungo. Mayu leaves the house and Shungo is under arrest.
Episode 11
Reika's inner self has taken over and uses her corporate muscle to separate Shungo from Ryōko in an attempt to restore his memory. After reminiscing with Reika, he's drugged and held captive (plus nearly raped) in order to keep him from Mayu. Irori and Tasuku then combine forces with Shungo's other friends in an attempt to rescue him.
Episode 12
Shungo races to find Mayu before she runs out of energy, only to run up against Hosaka. Mayu sacrifices herself to save him, only to be accidentally revived by the now complete Reika.
