For my own sake, all information here is based on the player choosing female Corrin. This section is basically an outline of what happens at the beginning of the game. Further sections branching off into the paths will focus around Takumi's role in the story.
Fire Emblem Fates takes place in a fictional world, focusing on two countries: Nohr, and Hoshido. Nohr is based on European style, while Hoshido is based on traditional Japanese style. These two countries are feuding, while the King of Nohr Garon, wishes to conquer and rule Hoshido. Both games, Conquest and Birthright center around the conflict between the countries. In Birthright, you side with your Hoshido family. In Conquest, you side with Nohr. However, the first six chapters of the game are the same for both, the split actually occurs at chapter 6. Below I will list out small summaries of what happens while leading up to the split.
The game begins with a battle between the Nohr and Hoshidan royals. This acts as a tutorial for the player, but is also a premonition of future events in the game. You start the game fighting alongside the Hoshidan royals, with your Nohr siblings attempting to 'rescue' you. From the very get-go, Takumi has a distinct hatred for the Nohr royal family (and people from Nohr in general). However, at this point of the game Takumi doesn't have any clear resentment for Corrin, and is willing to follow her orders and even jokes around a bit afterwards. This part of the story is the only time you will be able to use Takumi in Conquest.
These events turn out to be a dream, and a glimpse of the future. You wake up in Nohr, with your Maids and Butler waking you for combat practice with Prince Xander. It is revealed that you had been confined to a fortress in Nohr, and have been given the opportunity to leave. The power of the dragon veins are also explained in this chapter. Royalty of Hoshido as well as Nohr can use dragon veins in battles to achieve various effects that can give you tactical advantages. You are quickly whisked away to the capital city of Nohr to meet with the king. While meeting with King Garon, you are gifted with the sword: Ganglari. After this, you are to execute some Hoshido prisoners. At the mention of your name, one of the warriors (Kaze) expresses some surprise at hearing it, but does not elaborate on why. The kind-hearted Corrin (with help from Leo) spare the prisoners behind Garon's back, and the story continues.
While scouting a supposedly empty fort for King Garon, a fight breaks out which leads to Corrin and Gunter falling into the bottomless pit. While falling, Corrin discovers that her maid Lilith is actually a dragon. Corrin is then captured by Rinkah, who brings her to Hoshido. In Hoshido, it is revealed to Corrin that she is actually a member of the Hoshidan royal family. She had been kidnapped as a child when her father, King Sumeragi of Hoshido was murdered in an ambush arranged by King Garon. After rescuing Sakura and Hinoka from Faceless, Corrin learns of King Garon who had been sending the Faceless to Hoshido to terrorize and murder civilians and warriors alike. She is shocked and horrified at this revelation. Shortly after, Corrin meets Azura, a Princess who was kidnapped from Nohr in retaliation of Corrin's kidnapping.
Corrin is given an opportunity to meet her true family, including her Mother Mikoto. Mikoto greatly wants to bond with her daughter, but unfortunately, Corrin remembers nothing of Mikoto. Before the public announcement of Corrin's presence in Hoshido by Mikoto, we get to see Takumi in his natural habitat! Here, he is less than impressed with Corrin and clearly wants her gone. This Takumi also snaps at Azura when she calls him by his name, citing it as a privilege. After the public announcement goes awry and Corrin's mother is killed by the sword Ganglari, Corrin transforms into an ancient dragon causing a rampage and nearly killing Azura. When Corrin comes back to her senses and sees the devastation caused by Garon's plan, she is horrified. Takumi is overwhelmed with grief and anger for his Mothers death, along with the innocent bystanders. He outright rejects Corrin as his sister. Though these events are devastating, they do lead to Corrin obtaining the ancient sword Yato.
Following these events and with no time to grieve, Ryoma is quickly informed of Nohr forces gathering at Hoshido's border. It includes the Nohr royals, who have come to take Corrin back and conquer Hoshido. It is here, that your fate branches into two. You can choose between siding with Hoshido (Birthright), and returning to Nohr (Conquest).
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