During a joint United States-Japan military exercise on the island of Oahu, Hawaii, elite Titanostrider pilots Isami Ao of the Japan Ground Self-Defense Force and Lewis Smith of the United States Marine Corps are poised to face off in a friendly duel. Their competition is interrupted when a mysterious and powerful alien force launches a sudden, devastating attack on Earth. These invaders, known as the Deathdrives, pilot giant mechanical bodies and quickly overwhelm conventional military forces. Just as all hope seems lost, a massive, sentient, and flamboyantly heroic robot descends from the sky. Identifying himself as Bravern, he specifically requests Isami to pilot him, creating a psychic and synchronized link. Together, they manage to repel the initial assault.

Isami, a stoic and serious soldier, is deeply disturbed by the bizarre and intimate nature of piloting Bravern, who is prone to shouting his attack names and playing his own heroic theme music. Lewis, a charismatic and brave marine who has idolized superheroes since childhood, becomes intensely interested in Bravern but is rejected as a pilot. After the first Deathdrive, Superbia, is destroyed, its pilot is ejected—a young, mysterious girl whom Lewis finds on the beach and names Lulu. She initially cannot speak or care for herself but begins to rapidly develop physically and mentally under Lewis's care.

The series follows the formation of a multinational Allied Task Force to fight the global Deathdrive threat, with Isami reluctantly serving as Braverns sole pilot. The narrative explores the intense and often homoerotically charged relationships between the three central figures: the unwilling Isami, the enthusiastic and devoted Lewis, and the over-the-top, loving Bravern, who insists only Isami can be his pilot. A major narrative arc involves Isami being interrogated and ostracized for his connection to the alien robot, while Lewis acts as his staunchest defender and friend. As the Deathdrives, each embodying a different vice, are defeated, Lulu is revealed to be connected to them.

The story takes a dramatic turn when it is revealed that Bravern is actually a future version of Lewis Smith. After dying in battle in the original timeline, Lewiss consciousness was merged with his own Titanostrider and a Deathdrive, allowing him to travel back in time to save Isami and the world. This revelation redefines Braverns deep affection for Isami and his refusal to let anyone else pilot him. The climax involves a final confrontation with the leader of the Deathdrives, leading to a resolution where the original Lewis Smith is restored to his human form, but at the cost of Braverns existence as a separate entity.