"The Constant"(Season 4, Episode 5)
We always knew Desmond was different, but "The Constant" showed us how different. This is the first episode that incorporates neither flashbacks nor flash-forwards in its narrative. When Lapidus accidentally veers off course on the way to the freighter, Desmond, whose proximity to the hatch explosion has altered him, becomes unstuck in time, volleying between the versions of himself in 1996 and 2004. We learn that this form of time travel can cause one's brain to explode, which is why Desmond — with Faraday's help — chooses Penny as his "constant," a person who knows you in both time periods. Forget Jack, Kate, Sawyer, Juliet, Sun and Jin. Desmond and Penny's poignant, time-traveling affair solidifies their position as the heart of the show.