"Shiawase Haitatsu Taneko" centers around Taneko, a cheerful and determined delivery girl who works for the Happiness Delivery Service, a unique company tasked with delivering happiness to people in need. Each episode features Taneko encountering various individuals facing personal struggles, ranging from loneliness and self-doubt to unresolved conflicts with loved ones. Through her deliveries, which often take the form of heartfelt letters, small gifts, or symbolic gestures, Taneko helps these individuals find clarity, resolve their issues, and rediscover joy in their lives.

The series is structured as a series of vignettes, with each episode focusing on a different recipient of Taneko's deliveries. While Taneko is the central character, the narrative shifts to highlight the lives and emotions of the people she assists, creating a mosaic of interconnected stories. Her unwavering optimism and empathy serve as the driving force behind her ability to connect with others and deliver happiness in meaningful ways.

Supporting characters include Taneko's colleagues at the Happiness Delivery Service, who provide occasional assistance or comic relief, as well as recurring individuals whose lives are touched by her work. The series balances lighthearted moments with emotional depth, exploring themes of human connection, resilience, and the transformative power of small acts of kindness. The episodic nature of the story allows for a diverse range of scenarios, ensuring that each delivery feels unique and impactful.