Questions for the Penumbra serie

[Googolplex]

Hello,
I would like to buy the Penumbra serie.
But I have some questions, because I do not know what parts of the series.

There is a free game (Penumbra_Full_v1.1) by 2006. Then there's Penumbra - Overture (German name: Im Halbschatten) and Penumbra - Black Plague and Penumbra - Requiem.

What is the difference between these parts?

Overture is the first part of this serie?
Black Plague is the second part of this serie?
Requiem is the extension for Black Plague?

And what about the free version?

And begins the Story with Overture or can I play Black Plague directly?

Thank you very much
Googolplex

PS:
Sorry for my bad english - I 'm from germany. This text was translated by Google.