The video is a bit too short to guess what the plot is, but it does provide some context: the guillotine is a symbol of the French revolution, so this gives us a loose historical context. The trailer matches the screenshot that got leaked earlier, although I have to say that the static images looked much less impressive from a rendering standpoint, so this may explain why Ubisoft chose to release this video which provides a more positive frame of reference for the game’s graphics. Without knowing what the video runs on and what the screenshots were taken on, it’s hard to comment further.
Ubisoft made it clear that Assassin’s Creed Unity would come out for the Holidays 2014, so this means that the game has to be “done” by August or so. This is the typical development schedule for the holiday game releases. By now the game system should be a well-oiled machine and Ubisoft is expected to be mostly producing content and tweak the gameplay for this new AC episode.
The word on the street is that AC Unity will come for Xbox One and PS4, but also for Xbox 360, PS3 and Wii-U, so it is very possible that the original screenshots leak came from a “last-gen” console, while the new video shows “current-gen” rendering. See you in Paris!