"Aria the Animation" is set in the early 24th century on the planet Aqua, a terraformed version of Mars where water covers most of the surface. The city of Neo-Venezia, modeled after Venice, Italy, serves as the primary setting. The story centers on Akari Mizunashi, a young girl who moves to Aqua to pursue her dream of becoming an Undine, a gondolier who guides tourists through the city's canals. Akari joins the Aria Company, one of the three premier gondola companies in Neo-Venezia, and begins her apprenticeship under Alicia Florence, a skilled Undine and the company's president.

The narrative unfolds through a series of episodic vignettes, capturing Akari's daily life, her interactions with friends, and her growth as an Undine. Akari's cheerful and optimistic personality allows her to find joy in the small wonders of Neo-Venezia, from its serene canals to its vibrant festivals. She forms close bonds with her fellow trainees, Aika S. Granzchesta from the Himeya Company and Alice Carroll from the Orange Planet Company. Together, they navigate the challenges of their training while sharing lighthearted and heartfelt moments.

Aika, a proud and determined girl, often serves as a foil to Akari's carefree nature, while Alice, a reserved and prodigious Undine, adds a layer of quiet introspection to their dynamic. The trio's mentors—Alicia, Akira E. Ferrari, and Athena Glory—provide guidance and wisdom, each embodying distinct traits that influence their apprentices. Alicia's grace and kindness, Akira's strict yet caring demeanor, and Athena's gentle and artistic spirit shape the girls' journeys.

The series emphasizes the beauty of everyday life, blending slice-of-life storytelling with subtle science fiction elements. Themes of friendship, perseverance, and the appreciation of simple pleasures are woven throughout the narrative. The tranquil atmosphere of Neo-Venezia, with its picturesque landscapes and nostalgic charm, serves as a backdrop for the characters' personal growth and the deepening of their bonds.

As Akari progresses in her training, she encounters various residents of Neo-Venezia, including the mysterious Cait Sith, a cat-like entity believed to bring good fortune. These encounters often carry a touch of whimsy and wonder, reinforcing the series' gentle, reflective tone. While the plot lacks high-stakes conflict, its strength lies in its ability to evoke a sense of warmth and tranquility, inviting viewers to immerse themselves in the serene world of Aqua.

"Aria the Animation" is the first installment in the "Aria" series, laying the foundation for Akari's journey and introducing the richly detailed setting and characters that define the franchise. Its episodic structure and focus on character-driven storytelling create a soothing and contemplative viewing experience, making it a standout entry in the slice-of-life genre.