Takes place approximately 50 years after the original D.C. ～Da Capo～ series.
The story revolves around Sakurai Yoshiyuki and Otome & Yume, the two granddaughters of the original D.C. ～Da Capo～ protagonists Junichi and Nemu.

"This is actually the second manga based on Da Capo II in active serialization. The other manga, running in Kadokawa’s Comptiq magazine, is considered to be the “canon” adaptation of the story (i.e. it’s based directly on the game). As the title “Imaginary Future” suggests, this manga proposes an alternate deviation from the game’s premise, where fan-favourite Nanaka Shirakawa (speculated to be a descendant of the famous Kotori Shirakawa) is the story’s main heroine."

Every year the flowers bloom. Mysterious Cherry Blossoms blooming all over a crescent shaped island. That island is the island of Hatsune...

The main character who goes to Kazami Academy, Asakura Junichi, has the power to see the dreams of other people in his sleep. He was also taught magic by an old lady that allows him to create sweets.

One day, Junichi seems to have seen someone`s dream again while he was sleeping. Inside that dream, a girl who was a childhood friend of his appeared. But, he was woken up by his sister Asakura Nemu, forcing him to come back to his regular life. Nemu doesn`t actually have any blood relation to Junichi but their bond is deeper than other real brothers and sisters. So deep that it gives the illusion that they`re lovers.

It was another peaceful normal day waking up early in the morning and going to Kazami Academy. Together with childhood friend Amakase Miharu, the academy idol Shirakawa Kotori, the two sisters Mizukoshi Moe and Mako who prefer eating nabe on the school roof for lunch. Going through their day as usual, their childhood friend, Yoshino Sakura, who they thought had moved to America suddenly appears. It was a surprise for Junichi and Nemu. Sakura tells them, "I came to fulfill the promise we made when we were children...".

(description edited from AnimeNfo)

Nemu is back home.
What's gonna happen with Junichi's life after this reunion with his sister?
"Da Capo" in Italian means "From the beginning"... then will we see the end to this romance?