A young man has to live with a leech-like brain-eating parasite who secretes a highly addictive blue fluid into man's brain routinely and in return, the man must seek out human victims for the parasite so that the parasite can eat human brains.
Duane still has long hair, and Belial wasn't living the dream as he does at the end of Basket Case 3. That makes this a prequel, seeing as how all three Basket Case films take place immediately after one another.