While at the port one night, Marina Quiroga, an eligible single woman and sister to the city's police chief, unexpectedly surprises a man who is stabbing a woman. This will awaken within Marina her instinct as a detective, and with the help of her loyal butler Héctor, she will start to investigate the crime.

After the killer threatens Marina, Arturo places security in his mother's house to protect his sister as well as to stop her from continuing her investigation. But Marina and Héctor manage to escape and end up at Belle Epoque, a theatre where they believe the killer might be hiding.

The killer has managed to evade the police and Arturo is forced to recognise that there is in fact a killer on the loose threatening the general public. Meanwhile, Marina and Héctor discover a new clue that leads them to the identity of two women who are in grave danger. Searching for them, they end up in the Women's Quarters, where they confront the killer in an epic final battle.

After detaining the killer it seems that Marina will be forced to give up her dream of becoming a cop, but thanks to Castaño the case is reopened. Together they conclude that the killer is still on the loose and the cops propose a plan to catch him: host a masked party for the clients of the House of Water.

Now that they know where the killer's next victim is, they must get to her before the killer does. But just as Marina is beginning to gain her trust, the masked killer appears. Marina manages to save Margó and promises to give her protection. But Marina will soon discover that no one is who they claim to be and she shouldn't trust the people around her.

Marina discovers the killer's mask in Pablo's car and confronts him. During the struggle they end up crashing into a tree. Two days later she wakes to discover Pablo by her side, trying to reassure her that he is innocent. With the mask in their possession they figure out that only one person could have made it. But the man lives in the Cies and to reach him they'll need Castaño's help.

After discovering that the murders began thirty years before in Orense, Marina, Ramírez and Pablo go there to search for clues to the fleur-de-lis killer’s true identity; what they never realised is that all the while the killer has been closer to them than they ever suspected.