FelixMcClure,anOxfordUniversityprofessor,isstabbedtodeathinhiscollegestudy.NotonlywasheanexpertinhisfieldbuthewasoneoftheUniversity'skeyfund-raisers.TheinvestigationfocusesonBrooks,aone-timecleanerforMcClurewhoquithiscollegejobabruptlyafterastudent,MathewRodway,jumpedoutofacollegewindowhighondrugs.WhenMorselearnsthatBrooks'daughterwastheobjectofaffectionforbothRodwayandanotherstudent,hehasanotherpossiblesuspect.Brooks'wifealsoworkedforoneofMcClure'sfriendsandMorseuncoversaconspiracythathemaynotbeabletoprove.

Morseistakenseriouslyillatamuseumreceptionandishospitalized.Inthehospital,Supt.Strange,visitsMorsetopushhimtotakeearlyretirement,whileDr.Millicent'Millie'VanBurengiveshimherbookona140year-oldcourtcaseknownastheOxfordCanalMurders.Thecaseinvolvedthemurderandrapeofayoungwoman,MrsJoannaFranks,travelingbycanalboatfromCoventrytoLondon.Thecaseresultedinthreeboatmenbeingsentencedtodeathandtwoofthemhanged.MorsestartstoreadMillie'sbookanddreamsaboutit.Hesoonhasanumberofproblemswiththecase.　　Whyweren'tthethreemenalsochargedwiththeftWhydidMrsFrankstakeaboatinsteadofatrainthatwasmuchfasterandcomfortableandonlyslightlymoreexpensiveWhydidn'tsheabandontheboataftershecomplainedaboutlewdbehaviorofthecrewattheshippingofficeinBanburyWhydidshethendrinkand'socialise'withthecrewWhathappenedtoher'carpetbag'withwhichshearrivedontheboatbutwhichwasnotmentionedinthecourtcase,norwasitstoredinthearchiveswithher,almostempty,trunkThetrunkwasmarkedwithinitialsofherfirsthusbandwhohaddied.Howdidithappenthathershoeswerefoundontheboatbutnobodysawherreturntotheboatfromtheforest　　PCAdrianKershawdoessomebrilliantlegworkforMorse.Hestudiedhistoryandhasinvaluablebackgroundknowledge.Forinstance,hementionsthattheboatmenhadbadreputationbecausetheyworkedonSundaysanddidnotattendchurch.LaterachapelwasbuiltfortheminOxford.　　Thedeadwoman'sclothesandtheshoesfoundontheboataresubmittedtomodernforensicinvestigation.Theresultisthattheshoesdidnotbelongtothedeadwomanfoundinthewaterbecauseshewasmuchtaller.Theshoeswereneverusedintheforest.　　OnlyMrsFranks'husband,CharlesFranks,wascalledtoidentifythebodyofthedeadwoman.Heclaimedthat,whileherfacewasdarkenedanddisfigured,hefoundabirthmarkbehindherear.Theprosecutorwelcomedthatinformationthatonlyahusbandoraloverwouldknow.Theaccusedwerenotshownthebody.Thedefenseattorneymerelyclaimedthattheguiltofthethreemenwasnotproved.Allthreeaccusedclaimedtotheendthattheywereinnocent.OneofthemwasnotexecutedbecauseheembracedChristianityinprison.　　MorseinstructsKershawtoinvestigateifCharlesFranksbenefitedfromhiswife'sdeathand,indeed,hepocketed300poundsinsurancemoney.CaseclosedCharlesFranksmurderedatallwomenanddroppedherinthecanalaftermeetingJoannaintheforest.IfJoannaFranksjumpedofftheboatatthesametimeasthedeadbodyhitthewater,sheswamtothebankandjoinedherhusband.Thecouplechangedtheirnames,inthecaseof'CharlesFranks'asecondtimeafterhe'died'asJoanna'sfirsthusband.Thefourthboatman,ayoungsterwhowasnotcharged,wasprobablypaidbyJoannatogivefalsetestimony.　　MorsetravelstoIrelandandhasagraveofJoanna'sfirsthusbandexhumed.Thecoffincontainsbagsofsandandsomestones.

Ayearafterhermurder,thepolicere-openthecaseofYvonneHarrisonwhentheyreceiveananonymousletterimplicatingaburglar,HarryRepp,whoisabouttobereleasedfromprison.Supt.StrangeassignsthecasetoDSLewis,whoischafingatthebitawaitinghispromotiontoInspector.Morsehasbeenillandhasonlyjustreturnedtowork.HestepsonLewis'toesbyinvolvinghimselfinthecase,butasthebodycountrises,Morsefindshimselfinchargeofinvestigatingawomanwhohadmanyaffairswithmanydifferentmen.Whenhelearnsthatthedeadmenwereinpossessionoflargeamountofcash,hesuspectstheywereblackmailinghermurderer.