Sakurada Jun was in the second grade of junior high, but he dropped out of school because he had problems socializing with his classmates. After he quit school, Jun refused to go out, and spent all his time at home shopping on the Internet for weird or cursed items. One day, a mysterious box appeared in the middle of his room. When he opened the box, he found a sophisticated doll, which looked so real that it appeared to be alive...

[Source: AnimeNfo]

As summer break arrives for the students, Sakurada Jun is busily studying on his own in the library, making up for time lost when he had confined himself to his home. Meanwhile, his Rozen Maiden doll, Shinku, has been having troubling dreams of late. The pain and sadness of defeating Suigintou was overwhelming, despite the inevitable battle they waged. After waking suddenly from a nightmare, she finds Suigintou's companion spirit wandering; upon giving chase, she discovers that the seventh Doll — Barasuishou — has awakened. She now realizes that Suigintou's fate may very well befall the other Dolls, for the appearance of the last Doll signals the beginning of the end of the Alice Games.

[Source: Anime News Network]

After Shinku refuses a brooch Jun wanted to give her as a gift, Souseiseki explains Shinku's strange behaviour by telling Jun what had happened a couple of centuries before, when the Rozen Maiden dolls first met.

[Source: AniDB]

Jun Sakurada, a middle school shut-in refuses to go to school after being traumatized by an embarrassing incident at school. One day, he receives a letter asking “yes or no”, and chose “yes”, meeting the living doll Shinku and the events of the original Rozen Maiden series unfolding. However, what would have happened if he had chosen “no”? Although he overcame his shut-in nature, this Jun can’t find a place where he belongs. Then, Shinku suddenly appears in this world where “Rozen Maidens” aren’t supposed to exist; what will their new encounter bring?!

[Source: Crunchyroll]