"Ultra B" is a science fiction anime series set in a futuristic world where humanity faces an existential threat from mysterious alien entities known as the "B-Class." These entities, which emerge from a dimensional rift, possess immense destructive capabilities and are nearly impervious to conventional weapons. The story centers on a specialized military unit called the Ultra B Defense Force, tasked with combating these extraterrestrial threats using advanced mecha technology and strategic combat tactics.

The protagonist, Rei Ayanami, is a skilled pilot and a key member of the Ultra B Defense Force. Rei is characterized by her calm demeanor and exceptional combat skills, though her past remains shrouded in mystery, with hints of a connection to the B-Class entities. Alongside Rei is a diverse team of pilots, each with unique abilities and personal motivations. These include Kaito Tachibana, a hot-headed but fiercely loyal pilot, and Yui Hoshino, a brilliant engineer who designs and maintains the mecha units. The team is led by Commander Haruto Saito, a seasoned strategist with a deep understanding of the B-Class threat.

The narrative unfolds as the Ultra B Defense Force engages in a series of high-stakes battles against increasingly powerful B-Class entities. Each encounter reveals more about the origins and nature of the aliens, as well as the true purpose behind their invasion. The series explores themes of sacrifice, resilience, and the ethical dilemmas of warfare, as the characters grapple with the cost of protecting humanity. Key plot points include the discovery of a hidden facility containing advanced alien technology, the revelation of Rei's mysterious past, and a climactic battle that tests the limits of the team's unity and resolve.

Throughout the series, the interplay between the characters' personal struggles and the overarching conflict with the B-Class entities drives the narrative forward. The story balances intense mecha battles with moments of introspection, delving into the psychological toll of war and the bonds formed in the face of adversity. "Ultra B" presents a compelling blend of action, mystery, and character-driven drama, making it a notable entry in the science fiction anime genre.