"Refrain Blue" is a 12-episode anime series that blends elements of romance, drama, and psychological exploration. The story centers on Takashi Murakami, a high school student who struggles with recurring nightmares and fragmented memories of a traumatic event from his childhood. These dreams often involve a mysterious girl in a blue dress, whose identity remains unclear. Takashi's life takes a turn when he meets Mizuho Oikawa, a transfer student who bears a striking resemblance to the girl from his dreams. Mizuho is reserved and enigmatic, harboring her own emotional scars, which gradually intertwine with Takashi's past.

As the narrative unfolds, it becomes evident that both characters are connected through a shared tragedy involving a boating accident years earlier. This incident resulted in the death of Mizuho's younger sister, Naoko, who was Takashi's childhood friend. The accident left Takashi with survivor's guilt and suppressed memories, while Mizuho grapples with grief and a sense of responsibility for her sister's death. The series delves into their emotional struggles as they confront their pasts and attempt to heal.

Supporting characters include Rika Shirakawa, a cheerful and outgoing classmate who harbors feelings for Takashi, and Yuuya Asou, a close friend who provides a grounded perspective on the unfolding events. Rika's presence introduces a love triangle, adding tension to the dynamic between Takashi and Mizuho. The story explores themes of guilt, forgiveness, and the process of moving forward after loss, as the characters navigate their relationships and personal growth.

The pacing of the series alternates between moments of introspection and dramatic revelations, gradually piecing together the events of the past and their impact on the present. The narrative structure emphasizes the psychological toll of unresolved trauma, with the recurring motif of water symbolizing both the accident and the emotional barriers the characters must overcome. By the conclusion, Takashi and Mizuho begin to find closure, though the resolution remains bittersweet, reflecting the complexity of their experiences.