Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a new version of the tale of Perseus, updated for teenagers. Perseus still goes on a quest to save the world, fighting monsters, travelling to the underworld, and cutting off the head of Medusa, but this Perseus is a high school student named Percy with Justin Bieber hair. Instead of wearing winged sandals he wears winged Converse tennis shoes, he clicks a ballpoint pen to make a sword, and he looks at Medusa's reflection in an iPod instead of a shield. Medusa has also been given an upgrade. Instead of an ugly old woman, she's glamourous Uma Thurman wearing a black leather jacket with a black turban and dark sunglasses which she removes when she wants to turn someone to stone.
In movies Medusa has usually been animated either by drawings, stop-motion photography or CGI, or the actor wears a wig of rubber snakes (which we do see here once on a stunt double.) I've also seen an Asian snake woman movie with a woman who wears a combination of rubber and real snakes which are somehow attached to her wig. Sometimes Medusa is shown as half woman, half snake, but not here. This is the best Medusa I've seen so far - a human actress with fairly realistic CGI snakes on her head. (In "Alexander" Anjelina Jolie also appears as Medusa with CGI snake hair, but it's very brief and not as good.) Using a live actress also lets Medusa speak some dialogue and show some personality which I don't think had been done before. The snakes also seem to have personality - expressing Medusa's emotions along with her eyes and face.
The plot is the usual cliche of a ticking clock to the apocalypse until the hero saves the world at the last second. Percy goes off with two companions, his sassy sidekick Grover, a horny half-goat Satyr, and Annabeth, a pretty teenage girl who is the daughter of goddess Athena, and who is a smart and talented warrior. Their first stop on a long journey to get to the underworld is Medusa's lair at a garden shop in upstate New York. When they get off the Greyhound bus we hear AC/DC singing "Highway to Hell", which is a nice touch, even though the Greek myth underworld is really both Heaven and Hell. Medusa shows up in her fashionable turban and shades. Uma Thurman has fun with the character, trying to seduce the kids into looking into her eyes, complaining to Annabeth that her mother was the one who turned her beautiful hair into snakes, and becoming enraged when she tells Percy that she once dated his father. She tells Percy if he doesn't open his eyes, her hungry babies will open them for him. That's a new attribute for the head snakes, which usually don't do anything.
In another nice touch, Percy uses the shiny side of his IPod to look at Medusa's reflection, but even better, Medusa picks up his iPod and seems to be fascinated with her own reflection when Percy sneaks up behind her and cuts off her head. (I assume Apple paid plenty for the product placement, though teenagers today probably don't know what an iPod is.) The kids wrap up the severed head and bring it with them for later use.
They steal Medusa's pickup truck and drive to the Parthenon in Nashville. On the way, at a motel, we see Medusa's head stored in the bathroom sink which hinders Grover's bathroom abilities. Instead of using the head to kill a sea monster as Perseus does in the Greek myth, here Percy fights a multi-headed fire-breathing Hydra until Grover pulls out the head and takes off the shades and the Hydra turns to stone.
From Nashville they drive to Vegas for the third pearl. They steal a Maserati and drive to Hollywood where the entrance to Hades is next to the Hollywood sign. Hell is a fiery version of Hollywood which the ferryman tells them is the scrapheap of human misery full of lost hopes and dreams and wishes that never came true and where all lives end in suffering and tragedy. (I'll bet all the Hollywood veterans making the film appreciated this part.)
The Medusa head shows up one last time in a mid-credits scene. After Percy's nasty abusive stepfather is kicked out of his house, he goes to the refrigerator to steal a beer. He finds it locked with a warning sign from Percy which he disregards. He breaks opens the door and sees Medusa. The screen goes black and before the credits start again we hear the crackling sounds of him turning to stone.