Whilesurveyingatelegraphlinein1861,WesternUnionengineerEdwardCreighton(DeanJagger)isseverelyinjuredinanaccident.HeisdiscoveredbyVanceShaw(RandolphScott),anoutlawontherunfromaposse.Forcedtotravelonfootafterhishorsewashurt,ShawatfirstconsidersstealingCreighton'shorse,butchangeshismindandtakesthemanwithhim,savinghislifeddd
　　Sometimelater,followinghisrecovery,CreightonreturnstoOmaha,NebraskaandplanstheconstructionofatelegraphlinefromOmahatoSaltLakeCity,Utah.FacingconsiderableoppositiontothelinefromConfederatesoldiers,Indians,andoutlaws,CreightonelicitsthehelpofhissisterSue(VirginiaGilmore),foremanPatGrogan(MinorWatson),andassistantHomerKettle(ChillWills).
　　Lookingtoputhisoutlawpastbehindhim,ShawarrivesatCreighton'sWesternUnionofficelookingforhonestworkandishiredasascoutbyGroganwhoisunawareofhispast.Creightonrecognizeshimamongthemenandallowshimtostaydespitehissuspicions.CreightonalsohirestenderfootRichardBlake(RobertYoung),aHarvard-educatedengineerasafavortoBlake'sfather.ShawandBlakearebothattractedtoSueandvieforherattention,buttheirromanticrivalryiscutshortwhenconstructionofthetelegraphlinestartsonJuly4,1861.
　　Afterworkcommencesontheline,oneofthemeniskilledapparentlybyamysteriousbandofcattle-rustlingIndians.UnconvincedthatIndiansaretoblame,Shawridesouttoinvestigateandfollowstherustlers'trailtothecampofJackSlade,aformerfriendandcohort,whosegangcommittedthekillingdisguisedasIndians—thegangShawleftfollowinghislastbankrobbery.SladerevealsthattheyareworkingfortheConfederacytodisruptWesternUnionbecausetheybelievethetelegraphservicewillhelptheUnion.Shawridesawayandreturnstotheline.Notwantingtoturninhisformerfriends,ShawtellsCreightonthatalargebandofDakotaIndiansstolethecattle,andrecommendsthattheysimplyreplacetheherdandnotriskafightwiththeIndians.
　　Sometimelater,aconfrontationtakesplacebetweenmenworkingontheforwardlineandabandofdrunkenIndians.WhenoneoftheIndianstriestostealsomeequipment,anervousBlakeshootshim,ignoringShaw'sordertoremaincalm.AfterwordarrivesthatthemaincampisunderattackbyotherIndians,theWesternUnionmenrushbacktohelpwiththedefense.Atthemaincamp,Slade'smen,whoareagaindisguisedasIndians,stealtheWesternUnionhorses.ThecompanydiscovertherusewhenoneofthewoundedIndiansturnsouttobeawhiteman.ForcedtobuybacktheirstolenhorsesfromSlade,CreightonbecomessuspiciousofShaw'sinvolvement,especiallywhenheadmitstoknowingthegang.
　　SoontheU.S.ArmyarrivesandannouncestheIndianshavenowrefusedtoallowthetelegraphlinestogothroughtheirterritoryinresponsetoBlake'sshootingofthedrunkenIndian.Creighton,Shaw,andBlakerideouttoconvinceChiefSpottedHorsetoallowthemtobuildthelinethroughIndianterritory,eventhoughthemanBlakewoundedwasSpottedHorse'sson.CreightonisfinallyabletopersuadetheIndianstoallowthempassage,andworkcontinuesuntilthecompanyapproachesSaltLakeCity.
　　Sometimelater,ShawreceiveswordthatJackSladewantstomeetwithhim.OnthewaytoseeSlade,ShawiscapturedandboundbySlade'smen.SladesayshisgroupisgoingtoburndowntheWesternUnioncampandtheydon'twantShawtointerfere.AfterSladeandhismenrideoff,Shawescapesfromhisropesbutarrivesbacktoolatetopreventthefire.HehelpsrescuesomeoftheWesternUnionmenfromtheflamesandburnshishandsintheprocess.
　　Afterthefire,CreightonconfrontsShawforanexplanation,butShawdoesnotrevealwhatheknowsandisfiredbyCreighton.AsShawleavesthecamp,hetellsBlakethatSladeisactuallyShaw'sbrotherandthathe,Shaw,willfindSlade'sgangandstopthemfrominterferingwiththetelegraphproject.ShawridesoffandfindsSladeandhismeninanearbytown.Atthebarbershop,Shawconfrontshisbrother,whosegunisconcealedunderthebarber'ssheet.SladeshootsShawthroughthesheet.Shawfightsbackinspiteofhiswound,killingsomeofthegangmembers,andthendies.BlakearrivesandcontinuesthefightwithSladewhodiesfromhiswounds.Soonafter,theWesternUnionlineiscompletedandtheworkerscelebrate.WhenSuelamentstheabsenceofShaw,CreightontellsherthatShawcanhearthem.