Takuya Arima is a young student whose father, a historian who has conducted various researches, disappeared recently. During a summer vacation Takuya receives a peculiar package from his missing father, along with a letter containing information about the existence of various parallel worlds. At first Takuya doesn't take it seriously, but soon he realizes that he possesses a device that allows him to travel to alternate dimensions. Is his father alive, after all? If so, where is he? (Source: VNDB)

For high school student Tanaka, the act of being listless is a way of life. Known for his inattentiveness and ability to fall asleep anywhere, Tanaka prays that each day will be as uneventful as the last, seeking to preserve his lazy lifestyle however he can by avoiding situations that require him to exert himself. Along with his dependable friend Oota who helps him with tasks he is unable to accomplish, the lethargic teenager constantly deals with events that prevent him from experiencing the quiet and peaceful days he longs for. [Written by MAL Rewrite]

Tricked into assassinating his own prospective father-in-law, righteous samurai Raizou Kurima is overwhelmed with guilt. In order to reclaim his honor, Raizou swears vengeance on the lord who gave him those misleading orders. In his pursuit, he is aided by self-proclaimed merchant-of-all-trades Yuuen Usui, who is actually one of the Revengers—assassins who seek justice for the wronged and dispense bloody revenge on their behalf. Tragedy seems destined to follow Raizou as he becomes increasingly intertwined with members of the Revengers. As he begins working as a vigilante alongside them, the complicated web between the corrupt Japanese political elite, the unscrupulous Europeans, and the Revengers gradually reveals itself. The more entangled he becomes in these affairs, the more Raizou realizes that justice is as deceptive as the honor that he sought to redeem.

One day, Hitoyoshi Yokoya wakes up to a mysterious nameless girl dressed as a maid ringing his doorbell, asking to be his servant. With her former master referring her to the Yokoya household, she offers her expertise as an assassin. Intimidated by her background, Hitoyoshi tries to send her away; but when she leaves something behind, he rushes to return it to her and is nearly hit by a truck in the process. With her fast reflexes, the girl saves him just in time, and Hitoyoshi ends up taking her in. Ironically, the maid lacks any skills apart from the art of killing she has perfected since childhood. Nevertheless, Hitoyoshi welcomes the girl with open arms, encouraging her to leave her monochromatic past behind and experience a normal life. To seal the deal, Hitoyoshi names her Yuki, marking the beginning of her path to redemption and happiness.