Lee Barrett, a former intelligence agent, is approached by a man offering him a job to join a political organization against bioweapons. However, the man turns out to be an impersonator sent by Barrett's former boss, Eric Cavanaugh, to test his loyalty. Cavanaugh asks Barrett to investigate the murder of the security chief and the disappearance of Dr. Baxter, the head scientist at Station Three, a secretive bioweapons laboratory.
Barrett and Cavanaugh arrive at the laboratory and are advised by Dr. Hoffman to seal the laboratory using concrete. Hoffman reveals that there are two deadly bioweapons inside — a strain of botulinus that oxidizes after eight hours and a newly developed virus called the "Satan Bug" that could wipe out all life on Earth. Barrett enters the sealed laboratory and finds Dr. Baxter dead, with the vials containing the "Satan Bug" and botulinus missing.
Barrett receives a mysterious telegram from his former flame, Ann, who is also assigned to work with him on the investigation. They discover the dead body of another scientist from the laboratory and trace the suspect to Charles Reynolds Ainsley, a reclusive millionaire, and pharmaceutical tycoon. Ainsley threatens to release the viruses unless Station Three is destroyed.
After a demonstration of the botulinus in Florida, Ainsley threatens to release more in Los Angeles County unless the laboratory is closed. Barrett and Ann follow a lead to an abandoned car and find the missing vials. They are confronted by armed men who take them to Dr. Hoffman's home. Unbeknownst to the henchmen, they are being followed.
At the home, the henchmen realize they are being followed by security agents. In a confrontation at an abandoned gas station, Barrett convinces the henchmen to keep Ann hostage while he escapes the garage, which is set on fire. Barrett stops a passing car driven by Hoffman, who has double-crossed his own men. They struck a deal to locate the vials in Los Angeles.
As they are intercepted by security agents, Ainsley is arrested, and Barrett realizes the agents are actually Ainsley's security guards. He defeats them and confronts Ainsley again. Ainsley threatens to release the "Satan Bug", but drops it during a struggle. Barrett grabs it while taking control of a helicopter. Ainsley jumps out of the helicopter, and Barrett safely lands.
Meanwhile, Ann and the authorities discover the other vials hidden in the ice of a concession stand at the Los Angeles baseball stadium. Barrett contacts Ann and his superiors, and they safely disarm the bombs. Barrett prepares to land the helicopter, reflecting on how things have come full circle.
uniformed guard at Station Three, standing next to Raskin as they observe the landing of Agent Reagan's helicopter (Hubbard's character remains silent)
James W. Gavin
helicopter pilot who transported Agent Reagan to the compound and replies, "Yes, sir."
Mason, guard sitting at entrance to the compound, who replies to Reagan's question, "Six, sir. Eh… Doctor Baxter, Doctor Hoffman, Doctor Yang and three technicians."
cocktail waitress, at bar featuring bongo drums and piano music, who whispers in Barrett's ear and then he whispers in her ear (Remick co-starred in Sturges' The Hallelujah Trail, released two months after The Satan Bug)
clerk at Desert Air Motel who replies, "Oh yes, Mr. Barrett. You're in suite fifteen."
Michael Barrier
helicopter pilot, tracking the car carrying Barrett, Hoffman, Ann, Donald and Veretti, who says, "Getting a little difficult to keep 'em in sight. This is pretty rough country and it's closing in on the road."
SDI agent, one of Cavanaugh's men, driving the car that follows the vehicle carrying Barrett, Ann, Donald and Veretti, who says, "They've turned left on the Seco Road now. They—uh—slowed down and—well, for a moment we got awfully close to them."
SDI agent, one of Cavanaugh's men, in the passenger seat of the car that follows the vehicle carrying Barrett, Ann, Donald and Veretti. Doohan shares his scenes with William Bryant, but only Bryant has dialogue.
SDI agent who arrives at site of the jackknifed truck and starts a conversation with Donald, "Won't be long, he said. Did you ever hear a cop say anything else? It'll be hours."
SDI agent who stops at site of the jackknifed truck and starts a conversation with Veretti, "Look at all those cans and busted beer bottles. You'd think people wouldn't throw things out there like that."
police captain in charge of searching Dodger Stadium who says, "Hello! Yes, we're here looking. Nothing. Yes, we'll look in there. Of course, we'll look in there. Yeah, yeah, I know ..."
uniformed specialist in military radar, tracking the path of the helicopter carrying Hoffman, who says, "Now here's the helicopter, sir. We've kept everything away from it, but we think it's the one."
