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The trailer begins by reminding us of the last thing to happen: Agent May shot down Sarge in cold blood. Except... he seems just fine. Later footage shows Fitz and Simmons back in the lab examining the body. It's made clear that something's not right, and then Sarge wakes up.
Meanwhile, we get a glimpse of what mystery villain Izel is up to. She is approaching what looks like a Mayan temple, basically confirming SHIELD's earlier theories about her mythic origins. She's setting up some kind of ritual involving the monoliths and maybe even Sarge.
Character drama is also very present, with more emotional exchanges between May and Sarge. May seems to see the good in Sarge, believing a part of Coulson still exists. Sarge demands she stop using him as some sort of replacement. Mac gives a passionate speech about "the devil you know." Daisy continues to be unmoved by Sarge, right until the end when she has a chance to do him in. Surprisingly, it's Sarge himself screaming "Do it!"
As all of this happens, there's a repeated notion that Sarge is indeed Coulson. May seems to be on a quest to "save" him. Even Simmons makes mention of restoring the small part that still exists. For his part, Sarge believes himself to be connected to Izel, and that he must die to stop her.
Season six is wrapping up soon, so we'll get our answers before we know it. And maybe, just maybe, we'll get Coulson back.