Wah, wah, wah! Big, dumb Shrek has the hardest life ever. After finally beating the dragon and getting the girl, he's living the life he's always dreamed of—he's happily married to Fiona, the ogress of his dreams, and they have three adorable and lumpy green babies. They have good friends and a nice (well, nice for ogres) house and everyone loves them, yet Crybaby McWhinypants still manages to be completely miserable.
He's tired of the diapers and the playdates and all the responsibility that comes with fatherhood. He doesn't want to have to unclog the toilet; he wants to be a mean, scary ogre again. So after having a mental breakdown at his triplets' first birthday (brought on by the incessant demands of a fat, lollipop-licking brat), he runs away and signs a contract with the malicious Rumpelstiltskin, who promises to take Shrek back to the way his life was before he got exactly everything he ever wanted.
What a fucking jerk.
Shrek quickly learns that because of this utterly selfish move, no one else gets their happy ending either—Donkey is a slave to witches; Fiona is a bitter, loveless bitch; Puss in Boots is a fat and lazy domesticated wuss; and the land of Far Far Away is a miserable place ruled by Rumpelstiltskin, filled with grumpy people and slutty witches. It looks a lot like First Avenue in Belltown on a Saturday night. Shudder.
After seeing that things could've been so much worse, Shrek has to figure out a way to get back to the life he had before his selfish freak-out.
To do so, the movie follows the exact same formula of all the other Shrek movies—Donkey keeps singing Top 40 hits in his donkey voice, various fairy-tale characters make cameos, and Shrek learns a valuable lesson about honor.
Seeing as how this is the fourth installment in the series, and they haven't put much effort at all into freshening it up (because really, why would they—everyone loves Shrek), the only way you could possibly give a shit at this point is if (a) you're 7 years old and have the memory of a goldfish or (b) you're the parent of a 7-year-old with the memory of a goldfish who desperately needs to entertain their kid for 90 minutes because, like Shrek, you're battling some "I was so much cooler before I had kids" bullshit. But even then you might not enjoy it, because unlike Shrek, you don't get to sign a fucking contract to escape reality and relive your glory days—you're stuck on diaper duty. FOR-EV-ER. Ha-ha.