Instructions
If you see your neighbor anxiously pacing in front of their house, stop to talk to them. Your neighbor may tell you that they are upset because they can't find the keys to their house, and are locked out. They will ask you to help.
Although they don't know it, your neighbor has dropped the keys in the river. Grab your fishing pole and go fishing for keys.
Keep fishing until you catch the keys. You may need to fish for awhile, so don't give up. Look for a "Crucian Carp." The keys are similar to the shadow of this fish.
Once you've caught the keys, find your neighbor. Return the keys, and your happy neighbor will give you a reward, such as 600 bells, a "Gyroid," or a piece of furniture.