Helena Vasconcelos, a renowned philologist, receives an unexpected invitation from the National Museum of Lisbon. During an excavation in the catacombs of São Vicente Church, archaeologists discovered a 16th-century manuscript, written in Latin and covered with alchemical symbols. This manuscript, known as the *Codex Obscura*, seems to hold the secret of immortality, but it is incomplete. In the margin, there is a single phrase: *"Tempus est mutare"* - "It is time to change."
Intrigued, Helena becomes involved in the mystery but soon realizes that she is not alone in this quest. Miguel Ferrão, a former priest who distanced himself from the Church, has also been called upon due to his expertise in cryptography and ancient languages. Together, they begin deciphering the manuscript, a task that takes them on a journey across several European cities, each hiding a piece of the puzzle.
As they progress, they uncover a plot that dates back to the Templars and their connections to the Rosicrucian Order, secret societies that sought the secret of eternal life. Each place they visit - a monastery in southern France, a lost library in Germany, the crypts of the Vatican - holds new clues, all encoded in Latin and linked to esoteric texts forgotten by history.
However, Helena and Miguel's journey is shadowed by a clandestine organization known as *The Order of the Black Veil*, which also craves the power of the *Codex Obscura*. They are ruthless, and Helena begins to question just how wise it is to continue the search. The manuscript seems to hide more than just a secret about immortality; there is something dark, a force that manipulates the fate of those who come too close.
At the climax of the plot, Helena discovers that the true meaning of *"Tempus est mutare"* does not only refer to time, but to a change in the cycle of life and death. The price of immortality is not only physical but moral. The ending leaves the fate of Helena and Miguel unresolved, presenting them with an impossible choice: to sacrifice everything for a truth that could bring humanity's downfall.
