4A Games’ Metro 2033 is available for free for a limited time on Steam. Just go to the page here, log in, add to cart and check out. It is available for only 24 hours so be quick.
Based on the novel of the same name, Metro 2033 depicts a bleak post-apocalypse where people are living undergound Metro subways. The story centers around Artyom as he explores the depths of the Metro system and find the dangers lurking in there.
Initially released in 2010, the game later was remastered as Metro 2033 Redux. The game has a few sequels, Metro Last Light (2013) and the upcoming Metro Exodus releasing on February 22nd, 2019.