The militaristic Boazanian Empire, a society of humanoid aliens that cruelly oppresses its own hornless slave class, sets its sights on conquering Earth, which they call Terra Erthu. To combat this threat, a secret defense facility known as Camp Big Falcon is established. Its greatest hope lies with the super robot Voltes V, a massive fighting machine composed of five individual Volt Machines.

The story centers on the three Armstrong brothers. The eldest, Steve, is a natural but sometimes arrogant leader. His younger brother, Robert nicknamed Big Bert, is more analytical, while the cheerful and brave youngest brother is Jon, called Little Jon. They are joined by their skilled and courageous friend Jamie Robinson and the hotheaded but loyal Mark Gordon. Together, these five young cadets are selected to form the Voltes Team, piloting the Volt Machines to defend the planet.

The team’s formation is rooted in a deeper, hidden history. Years before the invasion, a benevolent Boazanian prince named Hrothgar was chosen to be the next ruler but was betrayed by the ambitious Zambojil, who seized the throne. Forced to flee, Hrothgar crashed on Earth, where he was rescued and given the name Ned Armstrong by a human woman, Mary Ann. They fell in love and had three sons: Steve, Big Bert, and Little Jon. While raising his family, Ned also helped Earth’s leaders build Camp Big Falcon and design Voltes V as a weapon against the empire he knew would one day follow him. He then mysteriously disappeared, returning to Boazania to continue his rebellion against Emperor Zambojil.

The initial invasion is led by the arrogant Prince Zardoz, the son of the usurper Zambojil. As Zardoz unleashes massive mechanized monsters known as Beast Fighters upon Earth, the newly formed Voltes Team achieves its first victory. The series follows their intense battles, the evolution of their combat techniques like the signature "Butterfly Return," and the growing bonds and rivalries among the five pilots. Their mission is complicated by the discovery that the Armstrong brothers possess superhuman abilities, a legacy of their Boazanian father.

A major narrative arc involves Zardoz infiltrating Camp Big Falcon under the alias "Manuel" to learn Voltes V's weaknesses, leading to a tense cat-and-mouse game with the base commander, Dr. Smith. Tragedy strikes when the boys' human mother, Mary Ann, is killed during a Boazanian attack, forcing the brothers to confront their grief and solidify their resolve. Later, the return of a man claiming to be their long-lost father, Ned, throws the team into turmoil, as they must determine if he is the real hero or an impostor sent to destroy them from within. The conflict escalates from a simple fight for Earth's survival to a personal and political struggle, revealing that Zardoz is the half-brother of the Armstrong boys, and that the true enemy is the tyrannical Zambojil. The Voltes team must ultimately decide the fate not just of their planet, but of Boazania itself.