A Half-Life 2 mod released this week adds locations, characters and story beats that Valve cut from the original game. Dark Interval takes all the tidbits of information we know about drafts of the genre-defining FPS and stitches them together into a standalone game. 

This week's release is just part one of the overall project, containing 11 levels in total including a revamped prologue, a reworked Kleiner's Lab section and a new locale called Manhack Arcade, where the player sees citizens of City 17 remotely piloting Manhacks (those annoying flying robots with twirling blades) to kill fugitives in the city.

The development team have filled in some of the gaps too, adding their own original work. "Dark Interval doesn't include original levels found in the 'leaked' version of Half-Life 2, and instead features brand new maps which were built from the ground up. It was decided that this was the only way to make them both stand out and be actually modern and not just modernised fix-ups," the creators said.

You don't need a copy of Half-Life 2 or any of its episodes to play Dark Interval, but you will need to download Source SDK Base 2013 Singleplayer, which is readily available in the 'Tools' section of Steam. Dark Interval can be downloaded here—that page has all the instructions you'll need to get it running. 

Here's some more screenshots from the mod: