Your objective is to stay alive until 3:33am. The problem is that during that time, a specter will try to bring your adventure to an end. There are several ways to know when he's close: for starters, the candle that your character carries around will go out when he is near.
In addition to that clue that he's around, you'll see where he has been because objects will have moved. For example, you might enter the kitchen and find the fire lit, or walk into a room and see that someone has turned the TV on. If that happens, you'll know that the 'Midnight Man' is not far away.
Midnight Man is a truly scary horror game. You're sure to be startled more than once if you try to make it through the entire night in the nightmare that is this game.