Here is the full list of all 46 Agatha Christie - there are Hercule Poirot: The First Cases achievements.
Complete chapter 9
Complete the denouement
Pass the denouement, flawlessly
Complete chapter 8
Resolve the blackmail and murder cases completely
Uncover a secret/memory from Archibald
Uncover a secret/memory from the Viscount
Make 150 deductions
Visit all locations in the game
Complete chapter 7
Examine all items in the staff-related areas
You have uncovered a secret/memory from Hugo
Complete chapter 6
You have uncovered a secret/memory from Inge
Make 125 deductions
You have uncovered a secret/memory from Ernesto
Inspect every window in the game
Uncover a secret/memory from Rehana
Return to the scene of the crime, alone, for a 2nd time
You have uncovered a secret/memory from Gedeon
Complete chapter 5
Uncover a secret/memory from Cassandra
Admire all paintings in the main hall
Meet all characters in the game
Inspect all items hidden in the snow
Uncover a secret/memory from Zakariya
Complete chapter 4
You have uncovered a secret/memory from Jacqueline
Make 100 deductions
Uncover a secret/memory from Margaux
Stop for a refreshment during a key moment
Complete chapter 3
Make 75 deductions
Complete chapter 2
Examine all books in the library
Make 50 deductions
Uncover a secret/memory from the Major
Complete chapter 1
Complete the prologue
Uncover a secret/memory from Elizabeth
You have uncovered a secret/memory from Florette
Make 25 deductions
Inspect all items in the prologue
You have uncovered a secret/memory from Luc
Uncover a secret/memory from Angeline
Complete the 1st objective