The Last Day was a Doctor Who miniepisode, released on the internet and serving as a prequel to the 50th Anniversary special, The Day of the Doctor. It was written by Steven Moffat, and starred Chris Finch, a relatively unknown actor, as a Gallifreyan soldier. The episode is only three minutes long, and is more of a sketch than a story.
The episode is shown from the point of view of a soldier fit with a headcam that records everything he sees. The headcam effects, including a heads up display and glitches, are part of the episode. The soldier is introduced to his station on Arcadia, the second city of Gallifrey. The city is protected by 400 skytrenches, which makes it almost invincible...unless a single Dalek should infiltrate through. That set-up probably gives a good hint of what happens next.
The episode, to me, is mostly a good chip in the unending and fierce debate about the merits of Steven Moffat as a show runner. I found the Christmas special, "The Time of the Doctor", which was part of the same arc as this episode, to be maudlin and overdone. Given a big budget spectacular with emotional moments and big stars, he writes something overwrought and confusing. "The Last Day", with only three actors, and only one set, is taut, scary, and uses effects and science-fiction concepts to advance a story and mood, rather than distract from it. To me, "The Last Day" is proof of how good Steven Moffat can be, when he isn't given too many resources to ruin his storytelling with.