After failing to pass a high school entrance exam, Wataru is unexpectedly whisked to Promised Island, where he meets four girls who turn out to be his sisters.
A chain of initially unexplained events leads Wataru to meet his eight other sisters.
Wataru and his sisters go on their first day of school, but Wataru is feeling the pressure to watch over them.
Wataru helps Hinako search for her teddy bear, even if that teddy bear only appears in her dreams.
The sisters are beginning to be suspicious about the numerous e-mail messages Wataru receives. So they each send their own.
Wataru plays a prince in a play. His twelve sister get into the act, too.
Simple fun with the laundry turned into play as the sisters dress themselves up as brides.
Wataru spends a day with Marie. Marie, however, becomes sick because of a downpour.
Wataru spends his initial summer days with his sisters.
Mamoru teaches her Big Bro Wataru how to swim.
Wataru and his sisters set out in an underwater tour in a submarine single-handedly constructed by Rinrin.
After a mishap involving Rinrin's submarine, Wataru and his sisters are marooned in an island with a very stylish beach house.
Wataru reminisces his time with his sisters so far.
Wataru proposes to the sisters that whoever has a request, he will grant them. Karen's request to have a lightbulb that illuminates a painting changed ends up with the two spending their time together.
The wind blows away Aria's ribbon (which her Mon Frère Wataru gave her). She is then accompanied by a strange old man to help her find it.
Kaho continues to fumble on her cheerleading routine. She however tries her best to cheer for her Brother Wataru in the autumn sports festival.
Haruka, being shy around her Beloved Brother Wataru, tries to ask him to take her out to the ball.
While exploring Chikage's mind/world, Wataru doesn't notice that he is having an out-of-body experience.
Shirayuki prepares some bento as a surprise for her Elder Brother Wataru.
Wataru celebrates Christmas with his sisters.
Rinrin introduces her latest invention to Bro Wataru—a gynoid clone of herself.
Wataru helps Yotsuba catch the thief known only as Clover.
Wataru's friend Akio arrives at the island for a visit, but his arrival makes Mami worried instead.
Akio tells Wataru that he had actually passed the exams. Will Wataru leave Promised Island because of this?
The sisters' house feels empty as they miss their brother and anticipate his return with all their hearts. Meanwhile, Wataru is back in Tokyo with Akio and is ready to forget his memories in Promised Island.
Against Akio's expectations, Wataru returns to Promised Island for good. The sisters become so happy for his return that they cherish every moment with him.
