"Regalia: The Three Sacred Stars" is set in the fictional kingdom of Enastoria, a nation recovering from a devastating war. The story centers on two sisters, Yui and Rena, who live a peaceful life in the city of Eridana. Their lives take a dramatic turn when a mysterious mecha, known as Regalia, emerges from the ground and attacks the city. During the chaos, Rena is revealed to be the long-lost Empress of Enastoria, and Yui discovers her own connection to the Regalia, powerful ancient machines tied to the kingdom's history.

The Regalia are ancient weapons of immense power, each linked to a specific pilot. Rena and Yui become pilots of two such Regalia, named Astrarise and Altarise, respectively. As they grapple with their newfound roles, they uncover the truth about their past, including the fact that they were once part of a trio of sisters who ruled Enastoria. The third sister, Ingrid, is revealed to have been lost during the war and is now a key figure in the ongoing conflict.

The narrative explores themes of identity, family, and the consequences of war. The sisters must confront their shared history while battling enemies who seek to exploit the Regalia's power. Among these adversaries is the organization known as the "Licht Regalia," which aims to seize control of the ancient machines for their own purposes. The story also delves into the political intrigue surrounding Enastoria, as factions within the kingdom vie for power and influence.

Throughout the series, Yui and Rena are supported by a cast of allies, including Tia, a skilled mechanic and childhood friend, and Sara, a mysterious girl with her own ties to the Regalia. The sisters' bond is tested as they face numerous challenges, including battles against other Regalia pilots and the revelation of dark secrets about their lineage. The story builds toward a climactic confrontation that forces the sisters to make difficult choices about their future and the fate of Enastoria.

"Regalia: The Three Sacred Stars" blends mecha action with emotional character-driven storytelling, focusing on the relationship between Yui and Rena as they navigate their roles as protectors of their kingdom. The series balances intense battles with moments of introspection, creating a narrative that explores both the personal and political dimensions of its world.