In this game, one player is an alien monster who has gotten aboard a spaceship, and the other controls the crew of the ship - as the back of the game says, it is "a simulation of one of those 'B' grade science-fiction movies of the 50s."
The crew has a number of different weapons they can use against the monster, but they don't know how effective each one will be and some of them can damage the crew or the ship. They can potentially repair the ship if it does get damaged. They also have to manage the ship's power, which must be allocated each turn to various systems.
At the start of the game the monster player gets to allocate points to various areas: strength, movement, concealment, life level, and defense against each possible kind of attack - which is why the crew player does not know how effective a weapon is until they try it.