How crucial is it to follow the story line?

[darkely]

Black Plague should then be played as new monsters are introduced along with tougher puzzles (and 10 strange artifacts throughout the game that unlock neat stuff).

Overture has more tougher puzzles. Of course, the computer room was hard, but the rest isn't so tricky and physics based as in Overture. Black Plague isn't so good as Overture, because it has less puzzles.
But I know a secret ending in Requiem when you collected all artifacts. What does happen in Black Plague?

I know that was addressed at The Rock Worm not me but in Amnesia there are 3 endings based on what you do in the end (trying not to say to much for those who have not played yet) correct? and for Overture and Black Plague do those have any alternate endings? Note I am still playing the Penumbra games so please no spoilers I just want to know if there was more then one ending.