Epic is a story about a girl called Mary Katherine aka MK (Seyfried) who goes to her dad's house in the forest. Her dad is a bit of a nerd who believes little people live in the woods but he hasn't even seen them.
Her dad has gone out to find the little people and MK follows him, chasing the dog. In the forest she runs into the Fairy Queen (Knowles) who gives her a magic pod and turns her into a little person. She is now as little as a blade of grass.
Then the Queen asks her to help get the pod to a special glow-worm so the next queen can be chosen. MK does this with a cheeky boy called Nod (Hutcherson), the Queen's protector, Ronin (Farrell), and a slug and a snail who are both really funny. The snail called Grub (Chris O'Dowd) is super-funny because he had tried to be a Leafman but he totally failed.
There are two sides in the forest. There are the Leafmen, who protect it, and the Boggans, who try to wreck it. The goodies and the baddies fight lots over who has the pod, and MK and her new friends must look after the pod to save the forest.
MK becomes best friends with Nod and they kiss. Girls will like that because they like yucky stuff. Boys will like it because there's plenty of fighting action and the little people fly around on birds that are agile.