"Gurren Lagann the Movie – The Lights in the Sky Are Stars" is a condensed retelling of the second half of the "Tengen Toppa Gurren Lagann" anime series, focusing on the final arc of the story. The film begins with humanity having reclaimed the surface world after defeating the Spiral King, Lordgenome. Simon, now a revered leader, and the rest of Team Dai-Gurren face a new threat: the Anti-Spiral, an alien race determined to prevent the overuse of Spiral Energy, which they believe will lead to the destruction of the universe.

The Anti-Spiral's forces launch a devastating attack on Earth, forcing humanity to retreat underground once more. Simon, alongside his comrades, including Yoko, Rossiu, and Viral, must rally their strength to fight back. The narrative explores themes of perseverance, unity, and the consequences of unchecked ambition. Simon's leadership is tested as he grapples with the weight of responsibility and the loss of his close friend Kamina, whose legacy continues to inspire him.

The film accelerates through key events, including Rossiu's temporary betrayal due to his fear of the Anti-Spiral's warnings, Simon's imprisonment, and the eventual reunification of Team Dai-Gurren. The climactic battle sees the team piloting increasingly massive mechs, culminating in the creation of the galaxy-sized Tengen Toppa Gurren Lagann. The final confrontation with the Anti-Spiral is a spectacle of determination and sacrifice, as Simon and his allies push the limits of Spiral Energy to protect the future of humanity.

The story concludes with Simon and Nia, his love interest, sharing a bittersweet moment as Nia, who is revealed to be an Anti-Spiral creation, fades away after the Anti-Spiral's defeat. Simon, now a humble driller, chooses to live a quiet life, leaving the future of humanity in the hands of the next generation. The film encapsulates the series' core themes of hope, defiance, and the enduring human spirit.