MorseandLewisinvestigatethedeathofLauraPoindexter,anAmericantouristwhoispartofanexclusivetourgroupvisitingOxford.Missingfromherpersonaleffectsisherjewelry,includingtheWolvercoteTonguewhichshewasgoingtodonatetotheAshmoleanMuseum.Thepathologistdeterminesthatshediedofnaturalcauses,butMorsefeelsthere'ssomethingnotquiteright.ThemysterydeepenswhenTheodoreKemp,theAshmolean'sexpertontheWolvercoteTongue,isfounddeadandMrs.Poindexter'shusbandgoesmissing.

ItreturnssixmonthsaftertheshockingfinaleofSeason3,andtheteammustovercometheirgrieftotackleadangerouscrimewave.Thistime,theglovesarewellandtrulyoffforProfessorTinhislecturesandanewmusicalpursuit,romanceisverymuchaliveforhismotherAdelaideTempest(DelaTour),andthelinesbetweenprofessionalandpersonalbecomeblurredfortherapistHelenaGoldberg(Stevenson).Meanwhile,DIMaiyaGoswami(SunetraSarker)takeschargeofthepoliceforcewithseriesfavoriteDSDanWinters(White)fiercelyatherside.