"Sora to Umi no Aida" is a 12-episode anime series set in a near-future world where humanity has colonized Mars. The story revolves around a group of young girls who are selected to become part of the "Earth Defense Club," an organization tasked with protecting Earth from mysterious alien creatures known as "UMA" (Unidentified Mysterious Animals). These creatures emerge from the ocean and threaten both Earth and Mars, prompting the need for a specialized team to combat them.

The protagonist, Ichiko Oki, is a cheerful and determined girl from the countryside who dreams of becoming a pilot. Alongside her are other members of the Earth Defense Club, each with distinct personalities and skills. The team includes Mana, a reserved and analytical girl; Karen, a confident and competitive pilot; Marina, a kind-hearted and empathetic member; and Elly, a mysterious girl with a connection to the UMA. Together, they pilot advanced mechs called "Aquarions" to fight the UMA and uncover the truth behind their origins.

As the series progresses, the girls face numerous challenges, both on and off the battlefield. They must navigate personal struggles, team dynamics, and the growing threat of the UMA, which becomes increasingly complex as their true nature is revealed. The narrative explores themes of friendship, sacrifice, and the consequences of humanity's expansion into space. The story also delves into the mysteries surrounding the UMA, their connection to Earth's oceans, and the broader implications for both Earth and Mars.

The series balances action-packed mech battles with character-driven moments, providing a mix of excitement and emotional depth. The interactions between the main characters highlight their growth as individuals and as a team, while the overarching plot keeps the stakes high as the UMA threat escalates. By the end of the series, the Earth Defense Club confronts the ultimate challenge, leading to a resolution that ties together the narrative threads and provides closure to the story.