Mitsuyoshi Tada is a high school student and aspiring photographer who has convinced himself he will never fall in love. He carries a quiet but deep emotional wound from his childhood, when his parents died in a car accident. The only mementos he has left are his father's camera and the final photograph his father ever took. Since then, he has lived with his younger sister Yui and their grandfather, helping out at the family's coffee shop, all while keeping his heart closed off from romance.

One spring day, while taking pictures of cherry blossoms, Mitsuyoshi encounters a cheerful and slightly airheaded foreign girl named Teresa Wagner. She is lost, having become separated from her travel companion upon arriving in Japan. Mitsuyoshi helps her and brings her to his grandfather's coffee shop, where she is soon reunited with her red-haired, fiercely protective bodyguard and caretaker, Alexandra Magritte, who everyone calls Alec. In a series of remarkable coincidences, it turns out the hotel where Teresa and Alec are staying is right next door to the coffee shop, and the very next day they enroll as transfer students in Mitsuyoshi's class at Koinohoshi High School. Soon after, the two new students join the school's eclectic Photography Club, of which Mitsuyoshi is already a member, along with his narcissistic but loyal best friend Kaoru Ijuuin, the club's perverted president Hajime Sugimoto, and the serious class representative Hinako Hasegawa.

The story follows the daily adventures of the Photography Club as Mitsuyoshi and Teresa grow closer. Teresa's boundless energy and her peculiar obsession with a vintage Japanese television drama called Rainbow Shogun slowly begin to chip away at Mitsuyoshi's emotional walls. However, the lighthearted school comedy is built on a foundation of hidden secrets. While Mitsuyoshi deals with his past trauma and guilt, Teresa harbors an even greater secret: she is not just a regular exchange student. She is Teresa de Larsenburg, the crown princess of a small European kingdom, and she is in Japan for a brief, precious taste of ordinary life before she must return home to fulfill her royal duties. In the second half of the story, this truth can no longer stay hidden. Her elegant and understanding fiancé, Charles de Loire, arrives from Europe, forcing Teresa to confront her obligations. Faced with an arranged future she cannot refuse, Teresa and Alec suddenly return to their homeland without a proper goodbye, leaving Mitsuyoshi and his friends heartbroken and confused.

This event triggers the story's final, emotional arc. Realizing the true depth of his feelings for Teresa, Mitsuyoshi overcomes his greatest fears including a deep-seated trauma related to flying to travel to Larsenburg. With the unexpected help of Charles, who selflessly recognizes the genuine love between Mitsuyoshi and Teresa, he sneaks into a royal ball. There, under the stars, Mitsuyoshi finally confesses his love, and Teresa must make an impossible choice between her heart and her crown. The narrative concludes with a bittersweet but hopeful resolution about following one's heart and the courage it takes to fall in love.

The light novel, Tada-kun wa Koi o Shinai Teresa Wāgunā no Jijō (The Circumstances of Teresa Wagner), serves as a companion piece to the original anime story. Rather than retelling the entire plot, it focuses specifically on providing Teresa's perspective. The novel explores her internal thoughts and emotions, her life and duties in her home country of Larsenburg, and the feelings she develops for Mitsuyoshi, offering a deeper look into the heroine's side of this romance.