Cobalt Blue is the story of three childhood friendsâ€”Ryoko (Nagasawa Masami), Kazuya (Rachi Shinji), and Daisuke (Fukushi Seiji)â€”who grew up together on a remote island of Okinawa.
Ryoko's mother, Morishita Yukiko (Tanaka Misato), is a world-renowned pianist who arrives from Tokyo to recuperate from an illness.
She falls in love with a fisherman, Nakamura Ryuji, then marries him and gives birth to their daughter Ryoko shortly before passing away.
Two boys, Daisuke and Kazuya, are born the same year as Ryoko, and the three children grow up like brothers and sisters, but as they near adulthood, they each begin heading toward their own separate paths. Kazuya decides to remain on the island to become a fisherman, while Daisuke is accepted into an art school in Naha (on the main island of Okinawa), and Ryoko considers leaving the island to study nursing. But during the spring of their 18th year, Kazuya sings a song of love, "Tubarama," to Ryoko, forever changing the relationship between the three friends...