By day, Romeo Maki is a respectable man who loves bringing gifts to the local orphanage, and Juliet is a newly employed nun working at said orphanage. However, by night, they are Masked Man M and Lady S, popular professional wrestlers. Kick-Heart tells the story of what happens when this dynamic pair meet in the ring, the only place where they can express their hidden desires.

High school student Morisato Ibuki has a startling dream about kissing a prince on a white horse. The next day, the prince from her dream, Ebisu Naoto, shows up as a transfer student at her school and a guest at her house. He had indeed kissed her, in an attempt to wake her up in the morning, although he claims that she kissed him. Ibuki is smitten with Naoto, but he seems to have other commitments, to American football and to a mysterious girl called Kawahara Izumi. Meanwhile, Nitta, a shy boy in Ibuki's class, has a crush on her. Will true love eventually find a way?

As introverted colleagues at a travel agency in Tokyo, Takuya Oohara and Rika Honjouji both cherish their happily single lives. After work, home is where they feel most at ease: Takuya unwinds with his cat, while Rika studies various maps. However, an uncertain future looms over their treasured tranquility when their company plans to open a branch office in Alaska within one year and transfer an unmarried employee there. During a day off, Takuya and Rika accidentally meet and, despite never having spoken before, open up to each other about their relocation fears. Feeling encouraged by Takuya facing the same predicament, Rika suddenly proposes a daring solution—to pretend to be engaged for 365 days. Takuya accepts, but little do the two know that the path to marriage may, in many ways, change forever the very lifestyles they struggle to preserve.

One day, Hitoyoshi Yokoya wakes up to a mysterious nameless girl dressed as a maid ringing his doorbell, asking to be his servant. With her former master referring her to the Yokoya household, she offers her expertise as an assassin. Intimidated by her background, Hitoyoshi tries to send her away; but when she leaves something behind, he rushes to return it to her and is nearly hit by a truck in the process. With her fast reflexes, the girl saves him just in time, and Hitoyoshi ends up taking her in. Ironically, the maid lacks any skills apart from the art of killing she has perfected since childhood. Nevertheless, Hitoyoshi welcomes the girl with open arms, encouraging her to leave her monochromatic past behind and experience a normal life. To seal the deal, Hitoyoshi names her Yuki, marking the beginning of her path to redemption and happiness.