After completing the Echoes of the Past quest, you can automatically transport with Yennefer to Loften, where the Missing Person quest starts automatically. You can also meet with her there later. In this quest you must only talk to the women in the middle of the village. They will point you to Freya's garden North from the village. Follow the sorceress there.
Once you reach the garden, the quest will end, and another one will start: Nameless. When you arrive, the main entrance to the garden will be closed - follow the sorceress to the walls near the tree, there you will see a track that leads to the garden. Inside you will find wolves (level 14). After walking down, turn left and go forward until you see a lever. First activate the right one, then the left one - a hatch will open and you will be able to swim through the sewer to another part of the garden.
There you will find a well. If you examine the nearby house, you will meet werewolf Morkvarg (from the In Wolf's Clothing quest). You don't have to fight him yet, just jump to the well and you will find the body of the Coward. A moment later Ciri's story will activate - The Calm Before The Storm.
As Ciri, walk to the sauna after the conversations ends. Leave the clothes in the cloakroom by interacting with the drawer with a mirror. After new conversations you will receive an optional task of bathing in the river and drinking a potion - you will notice that both those actions increase the amount of your health that will not regenerate during combat!
The battle will start soon after you leave the sauna and go to the stables. You will face the warriors of Wild Hunt and their hounds. After defeating few of them you will be ordered to escape - do it because the demons will appear constantly. When escaping on a horse, you must still watch out for hounds on your way, try to avoid them.
Once you get to the end of the road, a cutscene will start that will end this plot. After a short conversation between Geralt and Yen the quest will end and act II will start with the quest Ugly Baby.