Jim and Artie ride to Willow Springs and discover that the town is seemingly abandoned. The agents go to the saloon and discover everyone inside is frozen in position. There’s no money on the gamblers’ table and the till is empty. They check the other buildings and confirm that everyone is similarly frozen and the money is gone. The bank has been broken open and a man is lying on the ground in front of it. Artie confirms that the man is alive and they realize that everyone in town is alive but paralyzed.
Later, Jim meets with Governor Marcus Hawthorne and describe the paralysis. Artie is busy working on an analysis and Jim reports that everyone recovers after approximately 48 hours. The only fatality was an older man and the cause isn’t clear. Hawthorne seems vaguely casual about the situation, paying more attention to his shave. Once the barber leaves, Hawthorne tells Jim that he became governor by offending some political figures who “sentenced” him to the governorship. The governor plans to do an outstanding and return to Washington in triumph. He gives Jim a wire from President Grant assigning the agents to investigate the paralysis robbery until it is solved. Hawthorne points out that Grant put him in charge and promises to nail him to the wall if they don’t solve the case promptly. Hawthorne’s aide Hobson comes in and tells them that they received an incomplete request for help from Sand Hill. Jim leaves immediately to investigate.
Artie goes to the morgue to examine the older man’s body. Dr. Vincent Kirby comes in and Artie thanks him for volunteering his assistance. The doctor complains about exhaustion after staying up trying to make sense of the paralysis. He shows Artie a tissue sample indicating that a toxic substance attacked the nerves. Kirby suddenly complains of paralysis and tells Artie to give him an injection. When Artie goes to get the doctor’s bag, someone knock him out from behind.
In Sand Hills, the robbers are looting the town under the supervision of their leader, Coley Rodman. When one of the men, Frank Doyle, considers shooting the sheriff, Coley reminds him of their orders. Doyle challenges him and Coley invites him to try him if he wants. When Doyle backs down, Coley tells his men to ride out. They’re unaware that one of the men they’ve passed is Jim, feigning paralysis. He rides off after them and follows them to a fortress in the hills. Come nightfall, Jim scales the wall and uses a chloroform vial to make sure a sleeping guard doesn’t wake up.
In the barracks, Coley and his men are counting the loot. The gang’s leader realizes that Doyle held back some of the loot while Jim watches from the attic above. Doyle denies it and Coley demands his wallet. The rest of the gang back Coley but one of them, Lafe, tries to defuse the situation and suggests that the office Doyle checked was short of cash. Coley picks up a bottle and offers it to Doyle, but then disarms him and tells the others to get his wallet. They confirm he took the money and Coley beats the outlaw and then shoots him. Doyle falls back against a support beam and Jim comes crashing down into the middle of the gang.
Jim fights his way clear and knocks out the lantern, and then runs outside and hides in a tower building. A woman, Anna, comes downstairs and Jim grabs her. She’s surprised that Jim isn’t one of the robbers and offers to hide him in the tower. Anna explains that the men are afraid and ushers Jim in and then locks the door behind him. Coley tries the door and leaves, and Jim discovers that he’s in a storage room with dozens of test animals.
Coley goes back to the barracks and the gang reports that they can’t find Jim. Their leader mounts a guard, figuring that Jim didn’t escape and will try to get out before sunrise. Lafe suggests that they leave their boss, the doc, and warns that nobody knows what he’ll do next. Coley tells the gang that he’ll handle the doc and invites anybody who wants to, to challenge him, and the gang goes to obey his orders.
Anna brings Jim food and explains that she needs help. She wants to get her father away from the gang and explains that he studied germ theory with Dr. Lister. Her father went to China as a missionary and met her mother, and helped heal hundreds. However, there was a plague and the ruler’s wife died despite Anna’s father’s treatment. As punishment, the ruler killed Anna’s mother and drove them out. Now her father breeds new bacteria, including a germ that can paralyze. He doesn’t know that Coley is using it to loot towns, putting the germ in the towns’ water supplies.
As Anna leads Jim out, she warns him that the gang will strike soon and plans to kill everyone in San Francisco with a new fatal germ if they have to. Jim promises to return for Anna and her father as soon as he spreads the warning. The agent then climbs over the wall and runs off but a guard spots him and opens fire. The gang rides after him and Jim uses an explosive pellet to distract them. Despite that, the gang tackles the intruder and hauls him back to Coley. However, the gang has captured Artie, who was lurking nearby.
Jim meets with Hawthorne and starts suggesting that they guard the water and food supplies and ask for Federal troops to quarantine the area. The governor is getting another shave and refuses to spread a panic based on Anna’s story. He tells Jim that Kirby vanished and Artie turned up with a concussion. Hawthorne has no idea where Artie went since he left to search for Jim near Sand Hills. Jim grabs the barber’s razor, shaves Hawthorne’s throat, and makes sure that he believes everything is telling him. Hawthorne agrees to do what Jim suggested and offers the agent some troops, but Jim says that he’s going back to the fort on his own.
The gang ties Artie to a table and Coley brings some of the doc’s hungry lab animals to encourage him to talk. He rubs bacon on Artie’s face and Artie tells him that he stumbled across the place. Coley doesn’t believe him and tells Artie that the animals got the new germ. Artie then claims that he shot a man and was looking for a place to hide, and he has no idea who Jim was. Coley doesn’t believe him but Anna comes in and objects to Coley using her father’s lab animals. He lets her take them and prepares to shoot Artie, but the doc comes in: Vincent Kirby. He explains that he staged the fake autopsy and paralysis once he learned about the inquiry.
That night, Jim sneaks up to the fort using a dirt-colored tarp for camouflage. He ascends the wall using pitons and knocks the guard off the wall. Coley hears the man’s scream and goes to investigate.
In the tower lab, Kirby shows off his new germs to Artie now that he has an appreciative audience. He explains that he has developed the most deadly bacillus known and it will bring him fame. Disgusted, he explains that when he did serve mankind, it cost him his wife and his reputation. Artie warns him about Coley but Kirby is confident that he can outsmart the outlaw. Jim bursts in and puts up a fight with Artie, but they’re quickly subdued. Kirby orders them but then stares in shock at Jim. Jim yanks off Kirby’s wig as Anna runs in and calls to Jim. She admits that she knows Jim and says that he’s there to help them. Shocked, Kirby believes that his daughter betrayed him. He prepares a syringe containing his new culture and tells Anna to inject Jim and Artie with it to prove her loyalty. Crying, she gives the agents the injection.
Once the agents are paralyzed, the gang stores the agents in the tower storage room. The gang leaves and Jim and Artie drop the pretense. They’re glad that Anna just pretended to inject them. Meanwhile, Kirby tells Coley that he has changed their plan and doesn’t share his obsession with money. Coley isn’t interested in wiping out thousands using germ warfare, but Kirby plans to go to San Francisco and wipe out the city’s population. In the morning they’ll have proof when Jim and Artie die.
In the laboratory, Jim has Artie lean against the door and draws a chalk outline around him. He then mixes water with a chemical thermite paste from his belt buckle and daubs it on the outline.
Coley goes back to the barracks and tells the gang that it’s time to get rid of Kirby. The gang agrees and Coley says that he has plans for Anna, and then sends a man to dispose of the agents.
Jim ignites the thermite and it burns a hole through the door. When the outlaw unlocks the door, Jim kicks the burnt wood in his face and goes to get the doctor while Artie rescues Anna. The agent bursts into Kirby’s lab and the doctor is shocked, assuming that his germ failed. Jim invites the deranged doctor to go with him and get help, but Kirby starts ranting about his colleague’s jealousy.
Coley and Lafe find the unconscious outlaw and realize that the agents have escaped. They run into Kirby’s lab and a fight breaks out. Kirby grabs a gun but Coley shoots him. He then shoots at Jim but wounds Lafe, and Jim disarms him. When he tries to go for his gun, Jim fires a warning shot and tells him to carry Kirby outside. Once they’re out, Jim tosses a lamp into the lab, destroying it.
Later on the Wanderer
, Jim and Artie receive a telegram from the public health service confirming there’s no danger of an epidemic. Artie assures her that her father’s invention may one day prove useful as an anesthetic and Anna hopes that he finally helped people as he once intended. Jim tells her that she needs to start a new life and they share a toast. However, Anna tells them that her fiancé, John Chang, will meet her in San Francisco. She assures the disappointed agents that they can be good friends and they pour a glass of champagne for John.