ðə dˈɑːktɚ lˈɜːnz ðætðə pˈiːpəl ʌv ɛndpˈɔɪnt pɹədˈuːs ɐ hˈoːɹmoʊn kˈɔːld kælˈɪsti, wˌɪtʃ hɐz sˈɪmɪlɚɹ ɪfˈɛkts tʊ ɐdɹˈɛnəlˌɪn, ænd ðə kˈɪlɚ hˈæzbiːn tˈeɪkɪŋ hˈɛdz tə ɡˈɪv hɪmsˈɛlf ɐ pˈɜːmənənt səplˈaɪ.
The Doctor learns that the people of Endpoint produce a hormone called Kallisti, which has similar effects to adrenaline, and the killer has been taking heads to give himself a permanent supply.