Festum had approached an enormous migratory fortress Tatsumiya Island one year before their attack begin. Alvis wasn't ready for fighting against Festum at that time, they decided to execute the Plan-L that some enlistees fight for 60 days without the supply on the separated part of the fortress called L-Boat as a decoy while the main body departs the current area. This story centers on a prototype Fafnar pilot Ryo Masaoka and other pilots and crew on L-Boat.