The Garden of Evil Affair - S1.E8

A doctor developes a serum that is able to transfer the mind of a dead person into the brain of one of his descendants. The doctor is murdered by members of a cult who kill by scratching their victims. The Cambodyses cult are fed small amounts of poison on a daily basis until they are immune to its affects. However, it renders their finger nails deadly weapons. The cult, Thrush, and UNCLE want the serum and the doctor's granddaughter.