A new Assassin's Creed: Unity trailer puts forth the intriguing possibility that you could be playing in multiple time periods including what looks like an alternate version of World War 2 Paris.
The new trailer gives little to no explanation for the action-packed piece of in-game footage which sees French Revolutionary Assassin Arno Dorian climb up the Eiffel Tower and then take on a number of fighter planes from the Second World War.
The narrator does give some hint of an explanation suggesting that it could be a glitch in the Animus, the software which allows people to go back in time and relive the lives of others.
Other games like Bioshock: Infinite and Far Cry have used this technique before, putting the player into an 'alternative reality' in order to add something new to the gameplay.
It also solves one of Ubisoft's biggest problems which is that it finally adds long-requested time periods that haven't as yet got their own full-length game.