After the intro scene, you find yourself in a battle.
Rout the Enemy
The objective of the battle in this chapter is to Rout the Enemy. First, choose your unit, then choose the enemy unit, and choose attack. The enemy unit has a red background behind its character picture and name, and there will be a battle icon above the enemy unit when you put the cursor on it.
After you kill the enemy unit, the enemies will make their moves, including drying up the river. Then your allies will make their moves.
Next, a Lancer will appear next to you. First, choose Takumi, then choose the Lancer, but make sure that Takumi stays diagonal to the Lancer. You and Takumi will attack together. Then choose Corrin and attack the Lancer to kill it. In future battles, be sure to have your units stay near each other so that they can help each other attack this way.