Like the island where so much of the action takes place, "Lost" giveth and it taketh away. Flashes of illumination for its viewers are routinely undone the next moment by bewilderment.

This is a game "Lost" devotees are happy to play — albeit fewer of them lately than there used to be. Four seasons in, the show demands even more of the viewer than it used to. But those who have stuck around know that rewards richly outweigh the frustration.

This was never more so than on Thursday's two-hour season finale. It might be the most rewarding, deliciously challenging episode in the history of this mystical ABC serial.

Spoiler alert: Read no further if you mean to watch it for yourself and want to preserve its surprises. There are many.

For instance, you get to see the man in the casket at the L.A. funeral home. Though identified at the end of Season 3 as Jeremy Bentham, he is shown to the audience at long last, lying in pasty-faced repose: none other than John Locke.

But how did Locke, who embraced life on the island, get to Los Angeles? And how did he die? Let the guessing begin.

Another mystery addressed: that recent crazy talk about "moving the island." Darned if it doesn't happen! But not like moving a couch from one room to another. This was moving from Now to Who-Knows-Where-Or-When.