Three years before enrolling at First High, Tatsuya Shiba is already living as his younger sister Miyuki's dedicated Guardian within the Yotsuba clan, treated by the rest of the family as little more than a servant despite his skill. A family trip to Okinawa is disrupted when a mysterious naval task force moves against the coast, forcing the siblings into a crisis that reshapes how Miyuki sees her brother and hints at the buried truth behind his flat affect and weak practical-magic scores.