The Deltas somehow manage to trick the Omegas into town and away from campus. The Deltas switch the Greek letters on their house and the Omega's house. So the Deltas' Moms and Dads are visiting them in the clean Omega house,while the Omegas' Moms and Dads end up in the messy Delta house. The Deltas switch the letters back before the Omegas return. Naturally, the Omegas never find out that this has happenned.
