Dare I?
To jump straight to the not-so-fun part: it's a 43.5GB download for Call of Duty: Advanced Warfare, and the promo only covers multiplayer. Also, it lasts until Sunday, February 22 at 1:00pm Pacific.
So you can't swoop in, oggle digital Spacey, and clear the campaign like I was planning to do.
I spent too long trying to figure out that instead of searching for a "play for free" button on the game's Steam page, I should have been looking for Advanced Warfare in my library, uninstalled, all along. Yup, there it is. If you're not seeing the title in your list, try updating the Steam client.
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