Bob Arctor is a junkie and a drug dealer, both using and selling the mind-altering Substance D. Fred is a law enforcement agent, tasked with bringing Bob down. It sounds like a standard case. The only problem is that Bob and Fred are the same person. Substance D doesn't just alter the mind, it splits it in two, and neither side knows what the other is doing or that it even exists. Now, both sides are growing increasingly paranoid as Bob tries to evade Fred while Fred tries to evade his suspicious bosses.