For the sake of convenience, the Player Character will be referred to by their canon name of "Robin" and with male pronouns.
Neighboring to the Archanea continent was another continent named Valentia, split between two kingdoms; Rigel and Zofiya. The two kingdoms were also embroiled in war, but they were finally united with the efforts of Alm and Celica.
Meanwhile, lurking beneath the Labyrinth of Thabes within Archanea, a mad alchemist named Forneus holed himself up in an effort to create the perfect life form and a way to bring the dead back to life. Using materials which include the blood of the Divine Dragon, Forneus managed to create a life form on the size of a fetus. It grew into a peculiar creature, having six eyes. As the creature matured, Forneus began to see through its mind and saw only hateful and destructive impulses. Eventually, Forneus decided that the creature could not be controlled and must be destroyed, but this creature, only dubbed as 'The Creation', proved to be too powerful and Forneus was slain.
The activity of the Creation reached the ears of both Alm and Celica. Gathering their most able men, they traveled to the Labyrinth of Thabes and witnessed The Creation and a lot of men risen from the grave. Combining their strengths, Alm and Celica seemingly defeated The Creation, but noted that its malevolent presence filled with dread continued to linger within the place, which would never fade away. For the moment, Alm and Celica stopped that threat and resumed their lives, but The Creation would return in a time far in the future.
Years after Ylisse's formation, The Creation would return, in form of a terrifying dragon, now dubbing itself the Fell Dragon Grima. The Divine Dragon Naga, however, were still around. With the help of the First Exalt of Ylisse, she fought against Grima and put it in a long slumber. However, before his slumber, Grima has attracted a cult of worshippers, calling themselves 'Grimleals' and formed the theocracy of Plegia, becoming rivals with the Naga-worshipping Ylisse and hoping for the return of Grima one day.
All seems well, at first. As Chrom helps Robin to his feet, however, a dark force suddenly takes over Robin's body, forcing him to attack Chrom. Mortally wounded, Chrom assures Robin that he is not at fault as he collapses upon the floor, dead...
On their way to Ylisse, however, they come across a village being ransacked by bandits. Chrom and his fellow Shephers rush in to fight them off. Robin joins the fray shortly afterwards. As it turns out, Robin is not only handy in a scrap, but has a knack for battle tactics. He and Chrom both lead Lissa and Frederick to victory against the bandits and drive them out of town.
When the dust settles, Frederick shares with the others that he noted a distinct accent among the bandits, suggesting they came from Plegia, a kingdom neighboring Ylisse's western borders. Plegia has long desired war with Ylisse, and is known to send bandits in the hopes of instigating conflict.
Robin is confused as to why Chrom and his companions are armed for a bunch of shepherds. Chrom explains that his "flock" is the people of Ylisse: they fight to defend them.
With the town saved, the Shepherds and Robin set out once more...
As the forest becomes engulfed in flames, more Risen appear. The Shepherds get more help from Sully, another ally of Chrom; and Virion, an archer who doggedly pursues Sully's affections.
After the battle, the young man introduces himself as "Marth", taking his name from the Hero-King of Ylissean legend. He warns Chrom that what they saw was but a prelude to a greater calamity before taking his leave.
As day breaks, the Shepherds and Robin arrive in Yllistol, the Halidom's capital. While making their way through the city, Ylisse's ruler, the Exalted Emmeryn, appears to greet the townsfolk. Lissa reveals that Emmeryn is her older sister, making her and Chrom princess and prince!
Chrom and the others meet with Emmeryn in the palace in order to relay news of Plegian bandits attacking villages and the appearance of the Risen, as well as Robin's induction into the Shepherds. Before they can discuss the matters at length, however, news arrives of more Risen within Ylisse's borders.
The group set out, and are joined shortly afterwards by Stahl, a cavalier with a big appetite; and Miriel, a studious if deadpan mage. As they make their way north, however, they are set upon by Riisen. They manage to fight their way past them.
As they continue their treck, the Shepherds come upon a wounded pegasus. Chrom's attempts to aid it are met with resistance from the winged stallion, but Sumia reveals her knack for caring for pegasi. She stays behind to tend to its wounds while Chrom and they others march on.
Eventually, Raimi relents: recognizing Chrom's claim to be of Ylissean royalty as genuine, she permits him and his Shepherds entrance into Regna Ferox.
In the tournament, Chrom once more encounters "Marth", who is fighting as a champion of Basilio, the west khan and current ruler. To Chrom's surprise, the sword Marth wields looks exactly like his own Falchion! Marth claims he had learned to fight from his father, and he and Chrom prove an even match for each other. In the end, however, Chrom emerges victorious, as does his Shepherds.
With Flavia now ruler of Ragna Ferox again, she officially agrees to lend soldiers to Ylisse's defense. They are greeted by Basilio and, in respect of Chrom's martial skills, lends his best soldier to Chrom's Shepherds: Lon'qu, a swordsman from the far-away island nation of Chon'sin.
Chrom and the Shepherds return to Ylisse to relay the good news to Emmeryn. There is little time to rejoice, however: Plegian soldiers had crossed the border into Ylisse and had taken Maribelle captive! What's more, the ruler of Plegia, the Mad King Gangrel, has claimed Maribelle crossed illegally into his kingdom! Chrom wishes to kill Gangrel and be done with him, but Emmeryn insists that she meet him at the border and try to parley. Chrom agrees to allow her to go, but insists he and his Shepherds escort her.
Before Maribelle could come to harm, she is rescued by Ricken, a young mage-in-training. Gangrel declares war upon Ylisse and leaves his captain, Orton, to deal with the Shepherds. The Shepherds succeed in fighting off Orton and his forces and, with Maribelle safe, return to Ylisstol to discuss what how to proceed with the coming war with Plegia.
Chrom and Robin are approached once more by Marth, who warns them that the Exalt was in danger of being assassinated this very night. To prove the veracity of his claim, he warns Chrom of assailants waiting to attack him at that moment. Marth and Chrom fend off the assassins, but Marth's mask is broken in the fighting, and it is revealed that "he" is actually a young woman!
An explosion rocks the palace shortly thereafter as assassins, including a reluctant thief named Gaius, break in. A taguel named Panne joins the fray, as well. Marth reveals her to be an ally as they fight to drive off the assassins, lead by a foul sorcerer named Validar.
With Emmeryn safe, Chrom seeks out "Marth". She takes her leave, content that Emmeryn's safety had averted the series of events that had lead to the realm's ruination. Back in the palace, arrangements are made to escort Emmeryn to an eastern capital for safety while Chrom and the Shepherds petition Regna Ferox for additional aid.
All the while, Validar finds himself in a dark void, where he is approached by "Grima"...
Emmeryn decides to return to the capital, in spite of the danger, to bring hope to the people, and orders the Fire Emblem taken to Regna Ferox for safety.
Within Ferox, Chrom wrestles with grief and worry. Sumia, in an honest effort to snap him out of his funk, punches him across the face. Flavia, witnessing this event, renews her commitment to Chrom to send him aid, as well as joining the fray, herself, with Basilio in tow.
Unfortunately, terrifying news arrives: Emmeryn has been captured, and King Gangrel intends to put her to death! Chrom vows to save her, and Robin agrees to lend his aid.
The battle swings in the Shepherds' favor, but their momentum is halted when Gangrel orders his archers to take aim at Emmeryn. He makes his ultimatum: relinquish the Fire Emblem, or the exalt dies!
Before Chrom could act, however, Emmeryn makes a bold gesture. She calls out to the Plegians and begs them to release their hatred and break free from the cycle of violence and vengeance as she does what she must to change the world. With her final thoughts being those of love for Chrom, Lissa, and her people, she then falls from the cliff and to her demise...
Before Chrom can exact revenge, Basilio warns him of more soldiers inbound, and the Shepherds make a reluctant retreat.