I've just finished the final level for the first time, and enjoyed the game epilogue. Steam dutifully informs me:
Achievement progress:
Storytime! (5/6)
Uh… six? I've got my five sixths through level endings… what's the sixth piece? Do I need Farce to get it? I'm still 23k short of that :/
Best Answer
From what I have been able to determine, based on previous times this question has been asked on the steam user forums, Farce is not required for this achievement. Which answeres your question there.
In most cases the users found they accidentily skipped some dialog or scenes, in the prolouge or at some point through the gameplay. The only way to get it in this case would be to play through until you find out what you skipped or missed and get the achievement that way.
While this answers your question, i wish i was able to help you narrow down what part you missed but I dont believe there is a way to check that.
Good Luck