"It's a Straight Line Once You Love!" centers on the romantic and comedic dynamics between two high school students, Yuni Kurita and Saku Ichijou. Yuni, a reserved and introverted girl, harbors feelings for Saku, her outgoing and popular classmate. Despite her shyness, Yuni is determined to confess her feelings, but her attempts are often thwarted by her own nervousness and Saku's obliviousness. The story explores their evolving relationship as Yuni navigates the challenges of expressing her emotions while Saku gradually becomes aware of her feelings.

The narrative is driven by Yuni's internal struggles and her efforts to overcome her insecurities. Saku, on the other hand, is portrayed as cheerful and somewhat clueless, often misinterpreting Yuni's actions. Their interactions are filled with humorous misunderstandings and heartfelt moments, creating a balance between comedy and romance. Supporting characters, such as their classmates and friends, add depth to the story by providing additional perspectives and contributing to the central relationship's development.

The manga emphasizes themes of personal growth, communication, and the complexities of young love. Yuni's journey from self-doubt to confidence is a key focus, as she learns to articulate her feelings and take risks. Saku's gradual realization of Yuni's affection and his own emotions adds a layer of tension and anticipation to the plot. The story progresses through a series of episodic yet interconnected events, highlighting the characters' emotional growth and the deepening bond between them. The tone remains lighthearted and engaging, with a focus on the relatable struggles of adolescence and the universal experience of falling in love.