The original turn-based classic X-COM: UFO Defense (aka UFO: Enemy Unknown) is available for free right now on the Humble Store. It’s giving away a Steam key for the Windows version of the game, which will run on modern systems.

The offer expires tomorrow at noon ET.

The title is part of a promotion on Humble driving eyeballs to their Humble Monthly package, described as a “curated bundle of games sent to your inbox” every month. Subscribers will get a free copy of XCOM 2, the 2016 sequel to the reboot by Firaxis games, right off the bat.

XCOM 2 has been back in the news because of the release of the long-awaited The Long War 2 mod. First announced over a year ago, the mod is a complete reimagining of the base game, and includes new weapons and classes, a revamped skill tree and a totally new worker-placement mechanic for managing the human resistance.