Previously On: A Matched Pair

After successfully defending Garreg Mach from Thales and the armies of Shambhala, the United Fodlan Strike Force moves quickly to join up with the Imperial army and strike the weakened Agarthans before they can recover. Byleth is initially unable to join them in this, having fallen into a restorative sleep after using Sothis' power to raise the fallen Kingdom, Empire, and Church soldiers. She awakens and makes her way to Shambhala in the nick of time, saving both Rhea and the entire Strike Force from a last-ditch effort by Thales to have his vengeance.

After this victory, word arrives that the Almyran army has crossed the border, spurred on by another faction of Agarthans, and all of Leicester prepares for a desperate last stand against the invading force as the Empire and Kingdom scramble to come to their aid. The Almyran army, at 100,000 strong, outnumbers the combined Fodlani forces by over 20,000 men, but the defenders are determined to hold the line until Byleth, who has left on a separate mission, sends word that they no longer have any cause to fear Agarthan retaliation.

Byleth, along with a small group consisting of Anacharsis, Shamir, Linhardt, and Lysithea, sneak across the Almyran border and make their way first to Susa, the Almyran capitol, and then into the desert to Themiscyra, Anacharsis' former home. Their objective is to destroy a signal station that would allow Periander, the leader of the eastern Agarthans, to use a system of submarines as relays to launch javelins of light on any target, no matter the distance, as well as to destroy the missile storage facility in the city and kill Periander. Their mission is time-sensitive: the longer they take, the more people die in the war between Fodlan and Almyra.

After the first full day of fighting, things look bleak for the Alliance. Count Gloucester and the men under his command are slaughtered by the unyielding onslaught, and it is only thanks to the timely arrival of forces from Adrestia and Faerghus that they are able to plug the gaps. During the second day of the war, Claude finds himself facing off against Edelgard and Dimitri. In an attempt to put up a convincing duel against Dimitri, Claude's sacred weapon, the Sword of Begalta, snaps in two, and a piece of the blade strikes him in the face. He loses his right eye, and calls for a retreat.

Meanwhile, Byleth and her group have successfully infiltrated Themiscyra, and are making their way towards Periander's ordnance vault...