"Planet With" is a science fiction manga series that blends action, comedy, and philosophical themes. The story centers on Soya Kuroi, a high school student with amnesia who lives with his caretaker, a mysterious cat-like creature named Sensei, and a maid named Ginko. Soya's quiet life is disrupted when a group of giant humanoid aliens, known as the Nebula Weapons, attack Earth. These aliens are accompanied by a faction of humans called the Sealing Faction, who claim to protect humanity by sealing away dangerous alien technology.

Soya soon discovers that he possesses a powerful mecha called a Paladin, which he uses to fight against the Nebula Weapons and the Sealing Faction. As the battles escalate, Soya learns that his amnesia is tied to a larger conflict involving the planet Sirius, which was destroyed in a war. The Sealing Faction, led by Generalissimo Benika, seeks to prevent Earth from suffering the same fate by eliminating all alien influences. However, Soya begins to question their methods and motives, especially as he uncovers more about his own past and the true nature of the Nebula Weapons.

The narrative explores themes of forgiveness, coexistence, and the consequences of war. Soya is joined by a diverse cast of characters, including his classmates Harumi and Nozomi, who become involved in the conflict, and the enigmatic alien princess Miu, who has her own connection to the Nebula Weapons. As alliances shift and secrets are revealed, Soya must navigate the complexities of the conflict while grappling with his own identity and the moral ambiguities of the war.

The story builds toward a climactic confrontation that challenges the characters' beliefs and forces them to confront the true cost of their actions. Through its intricate plot and character dynamics, "Planet With" presents a thought-provoking exploration of humanity's relationship with alien civilizations and the ethical dilemmas that arise in the face of existential threats.