Call of Duty 4 is one of the best-selling Steam games right now, but only because you need it to play an ambitious Modern Warfare 2 multiplayer mod launching this week

There’s a Call of Duty climbing the top sellers list on Steam right now, and it’s not Black Ops 6. It’s Call of Duty: Modern Warfare Remastered, the 2016 re-release of 2007’s Call of Duty 4. It’s currently number three on the best-selling games list (by revenue) in the US and hanging out in the top 10 worldwide, beating out the likes of Dota 2, The First Descendent, and Baldur’s Gate 3.

That’s a bit weird at first glance—the remaster going on sale for $20 somewhat explains the sales bump, but it doesn’t account for why this one not particularly active CoD remaster is outselling every other game in the series and most other games on Steam.

The real reason for the renewed interest is way more fun, and very PC gaming: There’s a long-awaited fan mod coming out this week that revives classic Modern Warfare 2 (2009) multiplayer on PC, and you need this remaster to get it working.

The project, called H2M, is coming out August 16. On the mod’s website, developers say H2M aims to recreate classic Modern Warfare 2 multiplayer while adding “a wide variety of maps, new progression, unique camos, remastered weapons, custom attachments, and more.”