The Fountain of Youth Affair - S1.E20

Mark and April arrange to meet an informant who claims a politically important woman is in danger at Rejuvena Isle. However, two killers with an odd gun shoot her first. When Mark and April arrive, the informant is dead, having expired from old age though a very young woman. It is discovered that Baroness Blangstead offers the fountain of youth, her wonder drug, to those who can pay her price: Official Secrets.