Plummer was sacked and fumbled the ball with about 15 seconds left. It was eventually recovered by the Broncos, but the ball stayed in bounds, so the clock kept running, and ran out before another play could be run. I think the officials conferred to confirm that the ball stayed in bounds, which led to the ref announcing, "That's the end of the game."
In my initial pass through this post, I wrote, "Plummer was sucked." I think that's no less accurate.