The below description is taken from LEGO.com.
“Bzzz. Bzzz? Bzzz!”
Until very recently, the Fly Monster was a regular housefly. Now that he’s been mutated into a half-fly, half-minifigure creature, he’s found himself a little bit confused. Part of the problem is that his mind is still pretty much that of a fly. The other part is that he doesn’t realize just how much larger he is than he used to be.
The Fly Monster is always running into windows, falling off ceilings, and getting stuck to pieces of tape. He’s also very afraid of spiders and flyswatters and flees at the sight of them, buzzing in insectoid terror. There is one good side to his new body, though: it’s a lot easier to get at all of that delicious rotting garbage now that he can lift off the trash-bin lids!