The 1999 OVA Legend of the Galactic Heroes Gaiden is the second adaptation to center on supplementary narratives from the Ginga Eiyuu Densetsu novels. It explores Yang Wen-li's achievements prior to the Battle of Astarte inside the Free Planets Alliance, along with continuing stories about Reinhard von Müsel and Siegfried Kircheis in the Galactic Empire.
Within the "Spiral Labyrinth" arc, Yang becomes highly regarded after orchestrating the civilian evacuation from El Facil. After receiving an informal double promotion and gaining more media scrutiny, he probes into the so-called Year 730 Mafia—a tight-knit circle of Alliance officers led by strategist Bruce Ashbey. Yang discovers a surprising assertion from Ashbey's late wife that the celebrated war hero was murdered, an allegation with serious political consequences.
In "The Mutineer," Reinhard and Kircheis are stationed on the destroyer Hameln II at Iserlohn Fortress. Having won the crew's respect, Reinhard takes charge when the captain suffers a critical injury, testing his ability to lead.
In "The Duelist," Reinhard, based in the Imperial capital Odin, hears about a conflict over mining rights concerning Dorothea von Schaffhausen, a friend of his sister Annerose von Grünewald. When Count Herxheimer proposes resolving the issue by duel, Reinhard steps forward to represent the Schaffhausen family.
"The Retriever" focuses on Count Herxheimer, who, fallen from grace among the nobility, intends to switch sides to the Free Planets Alliance, taking a Seffle particle generator prototype with him. Because the operation requires secrecy, Reinhard is given the cruiser Hässliche Entlein to retrieve both the defector and the advanced device.
"The Third Battle of Tiamat" begins as the Empire launches a massive assault on the Free Planets Alliance to mark Kaiser Friedrich IV's 30-year reign. In the clash between the Imperial task force and three Alliance fleets, Reinhard's rapid maneuvers prove decisive in turning the battle's tide.