April and Mark are looking for Professor Antrum who has been searching for Atlantis. Rumors indicate that crystals able to concentrate the power of the sun into a beam more powerful than a laser are hidden at the entrance to Atlantis. According to a cryptogram that entrance is on the property of an eccentric name LeGallows. Antrum is kidnapped by Thrush and recovered by UNCLE only to be re-kidnapped along with April and Mark.

April and Mark are in Venice to capture Horsch, a top-ranking crook. April manages to grab him and the three set off for the Yugoslavian border. When border guards board the train on which they are passengers, Horsch escapes. April gives chase, following him to a villa where she is captured and drugged. Meantime, Mark is trying to trace April and they eventually uncover a plot to loot the Vatican.

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.