"Triangle Heart: Sweet Songs Forever" is a three-episode OVA series that serves as a spin-off of the "Triangle Heart" visual novel series, which later inspired the popular "Lyrical Nanoha" franchise. The story centers on Kōtarō Takamachi, a high school student who lives with his younger sister, Miyuki, and their adoptive father, Shirō. Kōtarō is a skilled martial artist and works part-time as a bodyguard for a local security company. His life takes a dramatic turn when he is assigned to protect a famous idol singer, Lindy Harlaown, who is targeted by a mysterious organization.

Lindy’s arrival in Kōtarō’s town coincides with a series of attacks by shadowy figures wielding advanced technology. These assailants are revealed to be part of a group seeking to exploit Lindy’s unique abilities, which are tied to her singing. As Kōtarō and Lindy grow closer, their relationship becomes a focal point of the narrative, with Kōtarō’s protective instincts and Lindy’s determination to continue her career despite the danger driving the plot forward.

The series also explores the dynamics within Kōtarō’s family, particularly his bond with Miyuki, who harbors feelings for him that extend beyond sibling affection. This subplot adds emotional depth to the story, as Miyuki struggles with her emotions while supporting Kōtarō in his mission. Shirō, a former martial artist and detective, provides guidance and serves as a mentor figure, offering insight into the challenges Kōtarō faces.

The action sequences are interspersed with moments of character development, particularly focusing on Lindy’s resolve to use her music to inspire others, even in the face of adversity. The antagonists, while not deeply explored, serve as a catalyst for the protagonists’ growth, pushing Kōtarō to confront his own limitations and Lindy to embrace her role as a symbol of hope.

The OVA blends elements of action, romance, and science fiction, with a narrative that emphasizes themes of protection, sacrifice, and the power of music. The story concludes with a resolution that ties up the immediate conflict while leaving room for further exploration of the characters’ relationships and the broader world they inhabit.