Agatha Christie - Hercule Poirot: The First Cases

Story-related; cannot be missed.

Here is a walkthrough of the chapter, including everything needed for achievements.

Great Hall
Talk to Angeline
Talk to Archibald
Talk to Margaux
Talk to Gedeon

Mind Map - Major's Involvement
"Angeline knew..." + "Angeline disputes..."
"Zakariya was shocked" + "Zakariya was therapized"

Great Hall
Talk to Gedeon
Zakariya Demir (15G) unlocks
Exit out the front door in the bottom left corner to go outside

Outside - Note that none of these items are particularly visible, you just need to go near them until the interaction point pops up
Look at the faint footprints near the door you came out of
Look at the expensive lighter also near the door
Go down and then right and over to a bench to find garden tools to look at
Go directly across (all the way left) to the very edge to find a fallen bird feeder
From the fountain in the center, go down and slightly right to find milk bottles against the fence
Look at the front gate in the bottom left corner (this must be done last!)
Sunken secrets (15G) unlocks

Great Hall
Enter the Dining Room on the right side

Dining Room
Look at the two interactable windows along the upper wall
Talk to Cassandra
Use the door on the far right bottom corner to enter the Kitchen

Kitchen (only time you'll be in this room)
Look at the mounted animal head on the wall
Look at the cupboard with the open door
Look at the pots and pans
Talk to Rehana
Well-acquainted (15G) unlocks
Leave back to the Dining Room
Leave back to the Great Hall

Mind Map - Major's Involvement
"Cassandra believes the Major..." + "Angeline disputes the Major"

Great Hall
Talk to Angeline

Mind Map - Major's Involvement
"Cassandra wasn't happy" + "Gedeon paid it off"

Great Hall
Talk to Angeline
Talk to Archibald to be taken to the Major's Study

Major's Study
Look at the safe in the corner to see a sealed letter
Look at the photograph
Look at the empty envelopes
Loot at the other letter

Mind Map - Major's Involvement
"Angeline and Cassandra" + "Angeline attached to Major"
"The Major cared" + "Cassandra and the Major's relationship issues"
"Large sum" + "Ransom value"
"Major paid the blackmail" + "Major faked the blackmail?"

Main Hall Second Floor
Look at the three paintings on the left wall
Look at the painting on the back wall
Exquisite artwork (25G) unlocks
Go to the bottom right hallway and through the door at the end to the Storeroom

Storeroom
Look at the book on the right side of the room
Look at the children's toys on the left
Exit back to the Main Hall Second Floor
Enter Cassandra's Bedroom (door on the top wall)

Cassandra's Bedroom
Look at the window
Talk to Cassandra

Mind Map - Major's Relationship
"High interest rate" + "Major resided with the family"
"Angline and Cassandra" + "Cassandra claims Major didn't love her"

Cassandra's Bedroom
Talk to Cassandra
Exit back to Main Hall Second Floor
Go downstairs

Main Hall First Floor
Talk to Angeline

Mind Map - Major's Relationship
"Previous birthday gift" + "Major returned after Viscount's death"
"Major hasn't helped the house" + "Cassandra has no romantic feelings..."

Main Hall First Floor
Go upstairs
Enter Cassandra's Bedroom

Cassandra's Bedroom
Talk to Cassandra - Forceful, Suggestive, Forceful
Cassandra Van Den Bosch (25G) unlocks
Exit back to the Main Hall Second Floor

Main Hall Second Floor
Talk to Archibald
The Secret (25G) unlocks