Final Crisis was a DC Comics miniseries and company wide crossover that was published in 2008 and 2009. It was written by Grant Morrison, widely considered to be one of the best writers in mainstream (and alternative) comics. The series takes up the plot threads of a number of different DC Universe series, including Crisis on Infinite Earths, Zero Hour, Identity Crisis, Infinite Crisis and 52.
Describing the plot of Final Crisis is fairly easy, although it launches easily into an editorial. Darkseid, the most powerful villain of the DC Universe, decides to take over the world by brainwashing people. Also, in a reference back to the multiverse concept established in Crisis on Infinite Earths and later carried through in Infinite Crisis, the race of monitors, a type of space-time guardian group, are also having problems, and one of them has turned into a vampire. Or something. Actually, the parts of the plot that weren't terribly cliché were just incomprehensible. Basically, it comes down to another story where the universe is about to be destroyed. And since the DC Universe has been almost constantly ending since 2004 and Identity Crisis, there is not much that can be done further at this point.
I am a big fan of a lot of Grant Morrison's other work, and actually some of my favorite stories of his were the big, over-the-top, cosmic catastrophe ones. For some reason, however, this just fell flat for me, perhaps because, as mentioned, it has been done before. Perhaps some of the blame lies not with Morrison but with editorial decisions: the story seems cramped and rushed in places.
Perhaps the best thing about "Final Crisis" is that it is final, and after 5 years of mind-twisting stories and continuity reboots, the DC Universe will settle down, the dead characters will come back to life, and everyone will go back to fighting alien invaders and themed criminals.