Deep Silver seems to think a survival guide will help you get through the post-apocalyptic Moscow subway underground of its upcoming Metro: Last Light. We'll go with that, and introduce you to part three of the series focusing on jury-rigged weapons and gas mask filters.
The video talks about learning new weapons via trial and error, as traditional weapons are rare. I also highlights your wristwatch, which is apparently light sensitive and has a gauge to monitor you breathing should your gas mask start to fail.
Of course, you'll be on your own when the game is finally released on May 14 for PC, Xbox 360, and PlayStation 3.