"Sōkyū no Fafner: Behind the Line" is a three-episode OVA series that serves as a prequel to the events of "Sōkyū no Fafner: The Beyond." The story is set in the same universe as the original "Sōkyū no Fafner" series, focusing on the island of Tatsumiyajima and its inhabitants as they prepare for an impending conflict with the Festum, alien beings that threaten humanity's existence. The narrative explores the lives of the island's residents, particularly the young pilots who operate the Fafner mechs, advanced weapons designed to combat the Festum.

The OVA delves into the emotional and psychological struggles of the characters as they grapple with their roles in the ongoing war. Key characters include Kazuki Makabe, a skilled Fafner pilot, and his childhood friend Sōshi Minashiro, who serves as the island's core system operator. The story also introduces new characters, such as Rina Nishio, a young girl who becomes involved in the island's defense efforts. The interactions between these characters highlight the themes of sacrifice, duty, and the human cost of war.

The plot centers around the island's preparations for a major Festum assault, with the residents working to strengthen their defenses and train new pilots. The series provides insight into the daily lives of the characters, showcasing their relationships and the challenges they face both on and off the battlefield. The narrative also touches on the ethical dilemmas surrounding the use of Fafner units, which require pilots to merge with the machines, risking their humanity in the process.

As the Festum threat looms closer, tensions rise among the island's inhabitants, and the story builds toward a climactic confrontation. The OVA sets the stage for the events of "Sōkyū no Fafner: The Beyond," offering a deeper understanding of the characters' motivations and the world they inhabit. Through its exploration of personal and collective struggles, "Sōkyū no Fafner: Behind the Line" enriches the overarching narrative of the "Sōkyū no Fafner" series, providing context and emotional depth to the ongoing battle against the Festum.