The Map Chamber

1. Meeting Your Classmates

The Girl from Uagadou introduces Natsai Onai, a Gryffindor who transferred from Uagadou and whose questline follows Ranrok's influence outside the castle. Around this point you also meet Sebastian Sallow, Poppy Sweeting and Amit Thakkar.

Each of the four has their own quest chain running parallel to the main story. They are not optional flavour: Sebastian's in particular carries the game's most consequential choices, and Poppy's is how you learn to handle beasts.

2. The Undercroft

In the Shadow of the Undercroft is the first of the "In the Shadow of…" quests with Sebastian, and it opens a hidden room beneath the castle that becomes yours. The Undercroft is where you later gain access to spellcraft the school would not approve of.

3. The Room of Requirement

The Room of Requirement becomes your base. Deek, a house-elf, introduces the tools you will use for the rest of the game: conjuring furniture, the potion and herbology stations, and the Vivarium where rescued beasts live.

It is worth investing here early. Growing your own ingredients and brewing your own potions removes most of the reason to shop, and the Vivarium is required for the beast collection.

4. The Map Chamber

The act closes in the Map Chamber beneath the castle, where the story finally names its structure: four Keepers — Percival Rackham, Charles Rookwood, Niamh Fitzgerald and San Bakar — each guarding a trial you must complete.

From here Act II is a sequence of those trials, with the companion and "In the Shadow of…" quests threaded between them.