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