As Microsoft promised, we'll be getting free post-launch episodes for Halo 4's Spartan Ops in the coming weeks (maybe months?). Episode 2 is arriving this Monday, and to precede that we have some amazing CGI work. I have a funny story about that. When the opening cutscene for Halo 4 started, I really thought it was live-action. No shame.
Anyway, I couldn't honestly tell you what was going on in this trailer, as I'm only about an hour in Halo 4's solo campaign and haven't touched the co-op stuff yet. Apparently that weird artifact you found is taking control of systems on the UNSC Infinity, and you'll probably be heading back down to the planet to fix things. Yeah, I'm still clueless on what's going on.
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