Migo (CHANNING TATUM) is a yeti, who lives in an advanced society of yetis on the top of a mountain. And Migo knows the laws: that the mountaintop was pooped out of a giant bird, that it rests on the back of giant animals, that the gong must be rung every morning to wake up the giant burning snail so it crawls across the sky.
These laws are written in stone -- literally -- and taught to all by the Stonekeeper (COMMON). And among those stones is the law: There is no such thing as a Smallfoot.
But as Migo trains with his father, Dorgle (DANNY DEVITO), to be the next ringer of the gong, he overshoots his target (the only way to ring the gong is to be slingshotted into it, head first) and ends up outside the city. There he spies a small aircraft crashing, and inside of it -- a Smallfoot! Or, as we would say, a human.
When nobody believes Migo saw what he saw, Migo is exiled from the city to think about what he's saying, and ends up falling off the mountain.