Ten years after the "Book of Darkness Incident" (refer to Magical Girl Lyrical Nanoha A's). 19-year-old Nanoha Takamachi, Fate T. Harlowen, and Yagami Hayate join the Space-Time Administration Bureau in pursuit of their respective dreams, and become active as first-class magicians. During this time, the newly discovered ancient heritage "Holy Relic Crystal" (レリック) and the mysterious mechanical weapon "Resonance Exploiter" (ガジェッドローン) that appeared with this as the target appeared. In order to quickly respond to the situation that would later be called the "Holy Relic Crystal Incident", Yagami Hayate established a special unit dedicated to handling tasks related to the ancient heritage, "Mobile Section 6", considering the need to set up an independent unit that can respond quickly. On the other hand, time goes back four years. Subaru Nakajima was almost killed by a fire at the airport, but was saved by Nanoha. Because he admired Nanoha who was like his "ideal image", Subaru made the decision to "become a magician of the Administration" under the starry sky that night. Four years later, after some twists and turns, Subaru successfully became a new magician of the Administration, and challenged the magician's promotion test together with his partner Tiana Lanster from the training school days. Staring at the two people's exam from a distance were Hayate and Fate, who believed that the two would become the future stars of "Mobile Six Section" and paid attention to them, and Nanoha. As the three of them watched, with the voice of ReinForceII (Zwei), announcing the start of the test, the prelude of the "StrikerS" story was opened.