You begin the game in a nightmare and try to escape from it. A clock starts at midnight and whenever you enter for the first time into a new area ("room" or "screen") it advances by 6 minutes (you'll awake and win past 8 a.m.).
There are hoardes of enemies (ghosts, medusa's heads and many other nightmarish beings), some static and other moving along patterns. Collision with them kills one of your "lives" (lose them all and the game ends in defeat).
You may move around, jump and collect items (including guns and their ammo, but several enemies are unscathed by those... it's a nightmare, after all). You have room for up to three items plus a bunch of bullets, so there's some simple inventory management.
Achieving successful actions (like collecting certain items) you do not score points, instead the definition of your dream in the upper part of the screen "improves" (or rather worsen: it starts as "bad dream" and goes on, including "morbid trance", "nightmare" and "frightmare").