We open aboard the Milano (the Guardians of the Galaxy's ship) somewhere in deep space, as the gang is looking upon a scene of mass destruction. Before they can get a handle on what's happened here, something smashes into the windshield — and it's Thor.
Bruised, battered and unconscious, Thor comes aboard the Milano, and Mantis wakes him up. He's disoriented and grouchy: "Who the hell are you guys?" he asks the gang. Sometime later, presumably after everyone's gotten to know one another a little bit, Thor says as they come up on another destination (Earth, perhaps?): "Something's very wrong."
We see Loki, holding the Cosmic Cube, facing down an unseen foe.
"Death follows him like a shadow," a female voice says (probably Gamora's, since he's her dad — though it could be anyone) as we see glimpses of Marvel's ultimate mega-villain.
We see Loki possibly facing him down, holding the Cosmic Cube in his hand. (He's not looking too confident here.)
We cut to Spider-Man riding a school bus that's crossing a bridge when the hair on his arm suddenly stands straight up on end. It's the spidey-senses, and here we are seeing them for the first time.
"He's coming for us," says Tony Stark, and by now we know for sure he's talking about Thanos, whose giant ship is crashing to the surface of whatever planet we're on (if this is Earth, it already looks pretty ravaged and post-apocalyptic, but it could be elsewhere).
And now we hear that voice, a digitally deepened Josh Brolin: "'Fun' isn't something one considers when balancing the universe," he says, "But this does put a smile on my face."
Thanos then emerges and breaks into a sly grin.
And this is when the trailer breaks into fast-cut montage mode: Spider-Man swinging into action in his new suit. Captain America in a new beard and shaggy haircut. Thanos, with the top of Thor's head in his hand, looking ready to crack it like an egg.
For a big finale, Thanos wields the Infinity Gauntlet (which appears to still be short a stone or two). He reaches back behind him as if to throw something, and the sky ripples and cracks behind him — he's summoning some destructive force from the very fabric of time and space itself, calling down giant fireballs of doom. I dunno, you kinda had to be there.
"You can fight it ... you can run from it," Thanos said. "But destiny still arrives."