Keanu Reeves plays Bob Arctor, an undercover cop who develops a split personality after taking Substance D (so named for causing "dumbness, despair, desertion and death"). He lives in a flop house in the Orange County suburbs with a group of D-using roommates that include Jim Barris (Robert Downey Jr.) and Ernie Luckman (Woody Harrelson), the eternally bug-haunted Charles Freck (Cochrane) and Bob’s girlfriend Donna Hawthorne (Ryder), who is suspected of dealing.

Bob's code name is "Fred" and his supervisor is known as Hank. When Hank assigns "Fred" to follow Bob Arctor, our hero is basically being asked to keep tabs on himself. Since the secrecy of his identity is paramount, he walks around police headquarters in a full body covering called a “scramble suit,” which randomizes his facial features a thousand times a second, making him unrecognizable.

The maze of deceit deepens when Barris, becomes an informant against Arctor. But Barris is unaware that he's sending this information to the disguised Arctor himself. Downey Jr., in fact, is quite impressive as Barris, compensating for his face being partially obscured by animation with a large performance full of swinging arms and narcotic tics.