Fallout, the 1997 original (subtitle: A Post Nuclear Role Playing Game) is free on Steam today, in observance of the 20th anniversary of its launch.

Users have until 11:59 p.m. tonight PT (that’s 2:59 a.m. Oct. 1, ET) to add it to their Steam library. It’s for Windows PC only.

I know I’ve got a couple copies of it around here somewhere — it went free on GOG.com at the end of 2013, just before Bethesda took full control of the publishing rights — but hey, free game. And it’s right there in the Steam library. And for those who didn’t start with the series until 2008’s Fallout 3, this is a great time to see where it all began.