男主角海神航，是一个麻木不仁、只为学业奋斗的少年。故事一开始在烦嚣的东京，正是考试放榜的日子，航和他的同窗兼朋友灿绪去看成绩，一心想著胜利是理所当然的他，却发现榜上没有他的编号！原来是因为选错多项选择的答案了？于是学校考不上，而一直照顾航的管家，亦要离他而去！正当事情达到这样糟糕的时候，突然两神秘人出现，说恭喜航受推荐入读一间神秘学校，而该学校竟在一个神秘的岛上－－Promised Island...... 航本来万分不愿意，但因毫无选择，只好出发，到了岛上，发现处处透著古怪，除了不断看到貌似管家的老人，扮成不同角色在航的身边出现外，最骇人听闻的是，十二位美丽可爱的女孩，先后出现，还自认是航的妹妹！就是这样，航就在这岛上，和十二个不同性格的「妹妹」住在一起，揭开他人生中最华丽的一页...... 同时，另一位叫真深的女孩，冒认是航的第十三位妹妹，混入Welcome House中和航给妹妹们一起居住，藉以监察航的一举一动，她更时常和一神秘人作电邮通信，他们的目的是让小航回到毫无希望的东京。
妹妹公主 (季节 1)
My Graduation (集 1)
播出日期: 2001-04-04
运行时间: 23 min
概述: Wataru Minakami is sent away from home to Promised Island to attend high school there. He falls off the ferry and almost drowns, but he's saved by a girl who calls him "Big Brother!"
Onii-chan, We Love You! (集 2)
播出日期: 2001-04-11
运行时间: 23 min
概述: As Wataru gets used to life in the mansion, many mysteries await him. There's a blistering-hot kimchi gratin no one remembers cooking. Then, he encounters a monster out in the woods! And finally... he meets even more sisters of his!
Together with Onii-sama (集 3)
播出日期: 2001-04-18
运行时间: 23 min
概述: Wataru goes to school for the first time. During class, he fends off all his sisters vying for his attention... and has to do the same thing again at home! Tired of it all, he attempts to escape from the island.
Where's the Teddy? (集 4)
播出日期: 2001-04-25
运行时间: 23 min
概述: Hinako, the youngest of the sisters, is missing! She left a note that she's "going to look for Mr. Teddy Bear." Kaho asks Wataru to go look for her, and it ends up being quite an adventure for them both.
Mail with Aniki (集 5)
播出日期: 2001-05-02
运行时间: 23 min
概述: Hinako sees Wataru using his PC and decides she wants to send him an E-mail. All his other sisters join in, sending him updates on what's going on in their lives. But who is his mysterious pen pal, "Aiko?"
Onii-chan, the Prince (集 6)
播出日期: 2001-05-09
运行时间: 23 min
概述: Hoping to win a mysterious prize, the family decides to perform a play together. Wataru naturally ends up in a starring role as the prince. Then he has to decide which sleeping beauty to awaken with a kiss!
Season to Love (集 7)
播出日期: 2001-05-16
运行时间: 23 min
概述: The wind blows a sheet onto Sakuya, and Hinako tells her she looks like a bride. One thing leads to another, and all of Wataru's sisters end up making wedding dresses and staging an entire wedding with him.
Someday with the Two of Us (集 8)
播出日期: 2001-05-23
运行时间: 23 min
概述: Wataru goes to a bookstore, and the shy, frail Maria and her loyal dog Michael accompany him. They have a pleasant time in town together... but then the rain starts pouring and Maria falls ill!
Summer Came (集 9)
播出日期: 2001-05-30
运行时间: 23 min
概述: Summer has arrived at last! Wataru and the girls enjoy cold somen noodles, try on new swimsuits, and finally play in the mansion's pool.
Do your Best, Anii! (集 10)
播出日期: 2001-06-06
运行时间: 23 min
概述: The bright summer days continue, and Wataru decides to learn how to swim. The athletic Mamoru takes the lead in teaching him. Can he overcome his lack of talent?
A Secret Tour with Aniki (集 11)
播出日期: 2001-06-13
运行时间: 23 min
概述: The family goes on a trip, which turns out to be on Rin Rin's submarine! Wataru has a great time with all of them, but then disaster strikes...
Vacation is Love (集 12)
播出日期: 2001-06-20
运行时间: 23 min
概述: Wataru, his sisters, and Mami are stranded on a desert island! They fashion new clothes and continue enjoying themselves. But the island has a secret!
Summer with Onii-chan (集 13)
播出日期: 2001-06-27
运行时间: 23 min
概述: While writing a letter to his friend Akio, Wataru reminisces about his summer vacation and the time he spent with each one of his sisters.
My True Feelings (集 14)
播出日期: 2001-07-04
运行时间: 23 min
概述: Wataru goes shopping in town with Karen. They have a pleasant day, but then Karen gets lost in the rain while trying to reunite two kittens! Can Wataru find her again?
Aria's Ribbon (集 15)
播出日期: 2001-07-11
运行时间: 23 min
概述: Aria's ribbon gets blown away by the wind, and she goes to look for it in town, accompanied by a mysterious old man. Then Wataru goes to look for her as well when he notices her missing.
Kaho does her Best! (集 16)
播出日期: 2001-07-18
运行时间: 23 min
概述: Kaho, the cheerleader, is excited to cheer for her brother at the sports festival. But she gets depressed at her own clumsiness. Wataru will have to give her courage!
It's Honorable... *Poh* (集 17)
播出日期: 2001-07-25
运行时间: 23 min
概述: While in town with Haruka, Wataru has an escalator accident and sprains his ankle! Will this interfere with their dream of going to the dance together?
Eternal... Vow... (集 18)
播出日期: 2001-08-01
运行时间: 23 min
概述: Wataru has a strange dream, and wakes up as an invisible ghost! Can the occultist among his sisters, Chikage, return him to his body... or is she the one behind this?
It's a Lunch of Love (集 19)
播出日期: 2001-08-08
运行时间: 23 min
概述: Shirayuki brings Wataru a boxed lunch at school. Then she cooks more and more, until Wataru's stuffed full! He'll have to find a way to turn down her meals without hurting her feelings.
Christmas Love Destiny (集 20)
播出日期: 2001-08-15
运行时间: 23 min
概述: Christmas is coming! Everyone has fun decorating the mansion and choosing presents. Wataru's holy night with his sisters will be truly magical.
Me Two (^o^) / to Aniki (集 21)
播出日期: 2001-08-22
运行时间: 23 min
概述: Rin Rin builds a mechanical copy of herself to help take care of Wataru, but Mecha Rin Rin keeps breaking down. Wataru gives her what help and reassurance he can.
Ani-chama, It's Cheki! (集 22)
播出日期: 2001-08-29
运行时间: 23 min
概述: All sorts of things are going missing around the mansion! Yotsuba, girl detective, is determined to find the culprit. But what if it's one of them?
First Guest (集 23)
播出日期: 2001-09-05
运行时间: 23 min
概述: Wataru's old friend Akio comes to visit Promised Island! All his sisters are excited to meet their guest. Except for Mami...
Premonition of Good Bye (集 24)
播出日期: 2001-09-12
运行时间: 23 min
概述: Wataru finally has the chance to return to Tokyo, but he struggles with the decision. One by one, his sisters find out that he might leave...
I Want to See You... Onii-chan (集 25)
播出日期: 2001-09-19
运行时间: 23 min
概述: The sisters try to get used to life without Wataru. And back in Tokyo, Wataru tries to get used to life without them...
The Promised Island (集 26)
播出日期: 2001-09-26
运行时间: 22 min
概述: After being challenged by Mami, Wataru makes his decision. He returns to live happily with his sisters once again.
季
集 26
集 13
集 3