Irrational Games has published a video showcasing the first five minutes of BioShock Infinite: Burial at Sea.
It contains spoilers, obviously.
Burial at Sea is set in Rapture, the city in which BioShock and BioShock 2 were set. The story sees Infinites Booker and Elizabeth on the eve of the underwater city's collapse, so you get a sense of it in its full glory.
However, as Eurogamer's Richard Cobbett recently revealed following a trip to Irrational to play through the DLC, it's "absolutely a piece of BioShock Infinite content". Burial at Sea stars Infinite's Elizabeth in femme fatale garb and uses Infinite's vigors and gear, and was built from scratch in the BioShock Infinite engine.