"Time Travel Girl" centers on Mari Hayase, a high school student with a keen interest in science, particularly the work of her late father, who was a physicist. The story begins when Mari discovers a mysterious device in her father’s old laboratory, which allows her to travel through time. This device, powered by a rare mineral called Wakatite, enables her to visit pivotal moments in scientific history. Alongside her cousin Kōhei, who shares her curiosity, Mari embarks on a series of journeys to the past, meeting influential scientists and witnessing groundbreaking discoveries firsthand.

The narrative unfolds as Mari and Kōhei travel to various historical periods, including the 19th and early 20th centuries, where they encounter figures such as Thomas Edison, Alexander Graham Bell, and Marie Curie. Each episode focuses on a specific scientist and their contributions, blending historical events with the protagonists’ personal growth. The time travel element is not merely a plot device but serves to highlight the challenges and perseverance of these historical figures, offering a glimpse into their lives and the societal contexts of their work.

As the series progresses, Mari and Kōhei grapple with the ethical implications of their time travel, particularly the potential consequences of altering the past. Their interactions with historical figures deepen their understanding of science and humanity, while also revealing more about Mari’s father and his connection to the Wakatite device. The story balances educational content with character-driven moments, exploring themes of curiosity, legacy, and the pursuit of knowledge.

The main characters, Mari and Kōhei, are portrayed as relatable and inquisitive, with Mari’s determination and Kōhei’s supportive nature driving the narrative forward. Their dynamic evolves as they face challenges together, both in the past and in their present-day lives. The series maintains a steady pace, focusing on the interplay between historical accuracy and the emotional journeys of its protagonists, making it accessible to viewers interested in both science and character development.