This installment is the third entry in Tadao Nagahama's Romance Super Robot Trilogy, preceded by Chou Denji Robo Combattler V and Chou Denji Machine Voltes V.
Following the ruin of their home planet, the refugees from Barm set a course for Earth, seeking to negotiate a land purchase for their relocation. However, during these talks, the Barmian leader Leon is murdered by his subordinate Olban, and the Earth delegation is wrongly accused of the crime. Amid the resulting turmoil, Doctor Isamu Ryuuzaki, a member of the Earth team, is fatally shot.
After the talks collapse disastrously, the Barmians—referred to as "Valerians" in Starbirds—initiate a violent assault on Earth under the command of Leon's son, Admiral Richter. He unleashes numerous "Mecha Soldiers" against the planet's defenses. The sole hope standing between humanity and destruction is the transforming super robot Daimos, which employs karate techniques, and its pilot Kazuya Ryuuzaki. Everything shifts when Kazuya meets and falls for a mysterious girl named Erika, who is later revealed to be Richter's sister. Through their efforts to be together, Kazuya and Erika each come to understand that not everyone on the opposing side is wicked—and that their own side harbors its share of wrongdoing.