Which game is worth it?

[Kedjane]

Well, Overture is probably the best. The graphics and all are worse than Black Plague but you'll get the story right from the start and get to know the universe better. Black Plague has better puzzles and all - and a pretty mindblowing start compared to any part of Overture, but if you buy Black Plague first you'll have some trouble understanding what's going on and you'll get to know pretty much exactly what happened in the end of Overture - which is a great cliffhanger ending.
So Overture first, then Black Plague.