Based on the Portal: The Flash Version (http://portal.wecreatestuff.com/). This Mappack brings all of the 40 levels plus some more originals. The story takes place before the events of Portal and shows you the enrichment center from another perspective.
IMPORTANT: This mappack requires both Portal and Half Life 2 to be owned and installed in order to work. Just drop the content of the RAR file (the 4 folders) in ValveSteamsteamapps-youraccountname-portalportal and replace files if needed. Then load the game, go to Bonus maps and Load the the Mappack from the list.