Small details like their matched white gloves and inconsistent names clue us (and Anna) into the fact that the invaders are up to no good. Paul borrows one of Georg's golf-clubs and uses it to kill the family dog then, during a minor argument, to break Georg's leg. Paul announces a bet, that none of the family will be alive in nine hours time.
The story is deliberately archetypal, borrowing from The Desperate Hours, Last House On The Left and Straw Dogs as an isolated "normal" family are invaded by psychopathic outsiders who arbitrarily torment them, forcing them to fight back. However, writer-director Haneke has ambitions to make more than just an edge-of-the-seat suspenser.
It might well be accused of being too clever for its own good, and the ending isn't particularly radical, but this is a remarkable film. Haneke's earlier effort Benny's Video was about the numbing effects of media violence but adopted alienating devices that made it cold. Here, he plays the game far closer to the rules, encouraging identification with the victims (especially Lothat) and making villains (who seem to be students out for a lark) unforgettably creepy characters.