The two arrive at Shiloh and are informed by Trampas and Belden that a telegram was received warning them that the Shannon herd probably is carrying Spanish fever which had begun to kill off the cattle in Casper. The local ranchers meet in the saloon, with Emmett Ryker present, to see how to deal with this threat, as once infected the fever will wipe 90% of the Medicine Bow herds in 24 hours. Although none of the Shannon herd are apparently dying, the Virginian points out that many cattle are immune but can still carry the fever. The Virginian is delegated to ride out and meet the Shannon herd before it reaches the area. Trampas learns from Mark that the Shannon's need to sell the herd to pay off a mortgage but Mark feigns indifference now that his sister has sided with Slauson. The Virginian gets Belden to introduce a few head of Shiloh cattle among the Shannon herd to see if they come down with the fever. Then the Virginian, Belden, Trampas and Mark ride off to meet Slauson and Peggy. They tell them of the threat and ask them to go back and take the normal route to Fort Collins so they will not infect the local herds. Slauson does not believe them and thinks the area ranchers are looking for a bribe but agrees to wait another day while the Virginian consults with the local cattlemen.
They all leave except Mark who remains behind and ends up challenging Slauson in a gunfight and seriously wounding him. Hearing the shots, the Virginian returns and goes to get the doctor and the Sheriff. In town, Mark claims Slauson drew first, which was a lie, but his sister backs him up and even Slauson, who is romantically interested in Peggy, tells the doctor and Sheriff that was the case. Mark is released and says he is taking over the trail drive and will wait no more than the day before moving it through the area. The Virginian meets with the cattlemen and proposes they chip in to help the Shannon's with some of their cattle losses if they take the cattle through the other route but they feign poverty and decide to form a quarantine line of all their men to block the herd being moved through. At the Shannon camp, the Virginian tells them of the cattlemen's decision and Mark does not believe it, thinking it a ploy.
After the Virginian leaves Mark decides, over his sister's objections, to stampede the herd through the next morning. One of the Shannon hands (Edward Faulkner) reports he has found dead Shiloh cattle which confirms the herd has the fever but Mark proposes to stampede his herd through anyway even though it may kill 25,000 cattle in the area. Peggy tells Slauson what has transpired and he tells her she must tell the local ranchers but she will not as she is afraid Mark will get killed in a confrontation. Despite his condition, Slauson rides to Shiloh and warns the Virginian, who, along with the Shiloh hands, ride to the Shannon herd location. They manage to turn the stampeding herd and Mark confronts the Virginian. The Virginian is forced into a gun battle and kills Mark. The Virginian sees off Peggy and Slauson, to whom he has provided extra hands to help get their herd to Fort Collins by the other route.