You play Jake the Verminator, beginning with a bank loan and a big mallet. You adventure throughout a magical tree with about 250 areas (platformer rooms) in search of the raw materials from which to make more money - or, even better, manufactured equipment. Shops sells various useful items, but they're expensive.
Your main function is using a weapon to eliminate vermin pests - they hurt your character but eliminating them doesn't earn much reward. Other ghost-like "mugger" bad guys make life even more miserable because they can deplete health and also steal money.
If you get really desperate, you can go gambling or even take out another loan. Periodically, you end a "week" of play and gain some resources and health over the "weekend".