It is about destroying the zombies whose aim is to cut the electricity by attacking the city's transformer. The transformer's health bar decreases as the player kills zombies. When it is reset, the player wins. If most of the zombies in the game reach the transformer, the player loses.
While killing zombies, one of the 4 different weapons defined can be used. Bullets can be earned from the randomly generated bullet box in the game. The score of the health bar is increased by using the randomly generated health box in the game.
Details of the game are in the video below.
Youtube video: