While fishing has been used in the past mainly for purposes of survival, today it has sort of evolved into a sport, or at least a hobby. In Big Mouth Bass 3D you are simply playing as a fisherman who is enjoying the sun and the fresh air, all while trying to catch as many bass as he and his partner can to make some money.
However, the problem is that even though they heard money could be made fishing bass, the guys don't really know how to catch the fish, and it's up to you to take control of things and direct their boat, as well as their road, to catching as many bass as possible. Also, because things aren't really interesting without some kind of a challenge, in each level you are going to have a limited amount of time to help the guys out, not to mention that you'll have to catch a certain number of fish too.
In Big Mouth Bass 3D you need to use the arrow keys in order to move your fisherman from left to right. In order to cast your line you need to press and hold the downwards arrow, and release it when you want to reel the line back in.
You need to make sure that the hook of your rod touches the front of the fish so that they bite on to the lure, at which point you will automatically reel it in and earn $100 for the fish.