Pedro continues investigating the murder of Sagrario. Gaspar manages to locate Marino, the butler, and Pedro talks to him. The butler tells him about the terrible way Isabel Ruiz Vela died, but he keeps information about his suspicions about the relationship between Isabel and D Juan Bautista. What Pedro gets is the name of the other servants and the doctor who signed the death certificate. Marino also mentions that one of D Juan Bautista’s nephews visited the house the night when Isabel died, and he called her to talk and they had hot chocolate. This sets off Pedro’s alarm bells, so he doubts Isabel’s death is due to natural causes.
The information Pedro gets from Rosarito, the other servant, doesn’t add anything to what he knows. Next he goes to see the doctor. The physician tells him that Isabel died of an intestinal obstruction. However, the doctor is honest and explains his doubts. One of the things that he finds strange is that Isabel didn’t scream, waking the others, when she felt sick. The doctor says that the pain would be excruciating, so why then didn’t Isabel cry out for help? Another thing is if there was an intestinal obstruction, Isabel shouldn’t have been able to evacuate stools properly. And the last point is that when he was in the room, there was another smell other than the disgusting odour of blood and stools, and he later remembered that it smelt of salt. Then the doctor gives him the sample of the substance that he picked from Isabel’s nasal cavities. The physician admits he is old and doesn’t have the means of younger colleagues, so he advises Pedro to find another doctor who can help him.
Pedro goes to see his doctor, D Alejo, and hands over the sample. D Alejo agrees to help him, and when they move to a room in the back of his house, the doctor uses one of the mice he has in his laboratory. As soon as the mouse eats a pinch of the substance, the poor animal goes into convulsions and dies. Then D Alejo confirms what Pedro suspects. The substance shows that Isabel ingested something that was poisoned with mercury chloride, which is a powerful poison that leaves a person unable to use his vital functions and smells of salt. Since it is a poison, the doctor says that a prescription is needed to buy mercury chloride, and the most logical explanation is that whoever killed Isabel Ruiz Vela bought the poison without a prescription. So the next step Pedro is going to take is to find the chemist who sold the poison, and to do that, he gets the help of Gaspar and Jeronimo Hiniesta.
The twins know that Pedro is after them, but when he talks to them in a brothel the two brothers frequent, both twins deny knowing who Sagrario was and also being in the house the night when both Sagrario and Isabel died.
Pedro is getting really close, but I am really afraid for him because these two men are dangerous and won’t stop at anything. They will even be able to hurt Pedro’s family if they think that will stop him. It doesn’t help that one of the twins’ friends is the man who Leonor Solís, the sister of those siblings who were accused of incest, had an affair with and got pregnant. Pedro was not very soft with him, so he might want to hurt Pedro as well.