Main story of the Gryps Conflict
While the AEUG's Argama was navigating the lunar surface, it intercepted the Titans' Alexandria.
The Titans' Alexandria also intercepted the Argama, and in order to pinpoint its hiding place and avenge Captain Laila, Lieutenant Jerid Mesa sortied in the Galbaldy β, Captain Laila Mira Laila's favorite mobile suit.
Lieutenant Jerid had been warned by the Alexandria that he would only be conducting reconnaissance, but driven by his hatred for Captain Laila's downfall, he attempted to provoke the Argama on the lunar surface in an attempt to lure out the Gundam Mk-II piloted by Kamille Bidan.
Kamille Bidan was intimidated by Lieutenant Jerid Mesa's determination and...

So it was you! You're the one who shot down Laila! | おうせい/ゆうせい's Gunpla works