简介：TheDoctor,SarahandHarryarriveonadesolateandapparentlydesertedEarthtodiscoverthatagroupofshipwreckedastronautsfromahumancolony,GalSec,havebeenluredtherebyafakedistresscall.Oneoftheirnumber,Roth,tellsSarahofanalienconductinggruesomeexperimentsonhimandhiscrewmates.ThealienturnsouttobeaSontaran,Field-MajorStyre,whoiscompilingareportonhumanphysicalandmentalcapabilitiesasapreludetoaninvasionofEarth. TheDoctorchallengesStyretounarmedcombat.TheSontaranagreesbutisquicklyweakenedinEarth'sunfamiliargravity.HarrymeanwhileentersStyre'sshipandusestheDoctor'ssonicscrewdrivertoremovethevitalterulliandiodebypasstransformer,sothatwhenthealienreturnstheretorevitalisehimselfheisdrainedofallhisenergyanddestroyed. TheDoctorsendsamessagetotheSontaranfleet,warningthemthatwithoutStyre'sreporttheycannotinvade.…