biohazard 4 Incubate is a supplementary book serving as a prequel to Resident Evil 4. It is written as a journal from the perspective of one of the villagers, allowing the backstory to be explained more thoroughly. Each chapter comes with a picture in a woodcut style, which also appears in the Resident Evil 4 credits. A DVD disc is also offered, which summaries the game itself.
The first three chapters introduce the village as peaceful, having abandoned much of modern technology to protect their culture and maintain a simpler life. The writer is identified as Rodrigo, who by the local culture was named by his grandmother. He confirms himself to be the artist behind the drawings, and intends to use them to teach his future grandchildren what life was like in his day. During the sowing season, where the adult men and women begin work tending to crops. The men take on the heavy labour duties while the women assist with lighter tasks, and provide songs to keep their spirits high in the morning sun. Rodrigo's wife bakes bread with the grains harvested. He compares the growing of crops with the rearing of children, and feels that if they continue to plant the earth all will be fertile. Rodrigo describes the older children as having chores in the barn, raising the chickens and calves. Observing a boy helping with the milking, he writes that since most of the villagers are blood relatives they feel a natural bond and help each other.
In the fourth chapter, Rodrigo is invited for lunch by a friend during a holiday. The man is proud of his new job, having been hired to assist in excavations at the nearby castle. Rodrigo writes on the 1st Castellan, a folk hero who slew evildoers and preserved the peace in the village. The story is frequently told by grandparents to their grandchildren while the parents are out working. While Rodrigo maintains a respect for the 1st Castellan, he feels differently for Ramon Salazar.
The harvest season approaches, which is an important time for the village as they can sell their excess produce to the outlying towns for supplies. Among the things they like to purchase is gasoline for their trucks, the only shred of modern technology which also serves as their means of transport. The entire village helps put the harvest on the trucks ready for market.
The following day is the harvest festival, where they are expected to eat, drink and dance to celebrate one of the few days they have no work to do. A villager plays his acoustic guitar for the people, who also recite poetry. Rodrigo notices that the celebration is not going entirely according to tradition - the village chief, Bitores Mendez is nowhere to be found. As he is also their priest, Rodrigo brushes it off as important church work.
Rodrigo describes the villagers getting back to work, collecting chicken eggs; catching fish and nuts and bringing them back to the village. He describes filling up a horse-drawn wagon with supplies for the village; the horse is ridden by his brother, who lives with his family in the outskirts. Rodrigo reminds his brother's family to attend church service the next Sunday, as Father Mendez has an important sermon.
Rodrigo and many of the other villagers attend the church service, in which Father Mendez gives over to Osmund Saddler. Saddler tells the congregation fantastic stories which persuade them to join Los Iluminados and take part in the blood cleansing ritual. Injected with an unknown substance, they are said to be purified of their sinful blood and their souls saved. Rodrigo believes that villagers who do not take the ritual will not be truly happy. After the ceremony Rodrigo has lunch with another villager who did not join the cult. The "sinful man" becomes enraged at the cult's activity in the area, likely recalling the stories of his youth. Rodrigo sees the anger in the man as proof that those outside the cult cannot be happy, and wants him to join so he can be protected.
Later, the villagers begin forcing their family members to also take part in the ritual to save them. This, Rodrigo sees, is a just act as they will all be connected again (ignoring his prior entry which described everyone as a family). Rodrigo's brother is brought out, and calls to Rodrigo to save him from the ritual. Rodrigo refuses his help, believing that it is crucial that all villagers go through the ritual. Later, he asks Father Mendez and Lord Saddler about converting his brother's family; they agree with him that it is necessary.
When the villagers have finished their conversion ritual, they are told Los Iluminados' version of their folk tale on the 1st Castellan. In their version, the 1st Castellan was envious of the cultists' righteousness and persecuted them. Ramon, the current 8th Castellan is reinterpreted as a redeemed man, having freed the cultists. Rodrigo is disturbed by the story, as it proves to him his family had been lied to for centuries.
In the time after their conversion, villagers begin to feel ill. Rodrigo's wife begins coughing. Their daughter is stricken at the dinner table, and has a seizure. Her body is contorted as her limbs shake violently, and finally dies; her parents feel nothing at all, which bothers Rodrigo. He then begins suffering painful headaches which feel as if his head is being split open. His blood becomes hot, and with so much pressure he begins to bleed out and swear with words that shock himself. Rodrigo runs to his brother's house, where he finds his brother too has fallen ill by vomiting blood. In the pain he becomes verbally aggressive, the point Rodrigo covers his ears.
Over the next few days every child in the village dies. Rodrigo writes that more of the adults are becoming emotionally emptied. Rodrigo writes that on some days he doesn't eat at all, and walks around the village for no reason, murmering and casually swearing before returning to bed. Later he too begins vomiting blood.
Father Mendez, in his capacity as village chief, issues a notice to the villagers to keep outsiders away from the village in order to protect Lord Saddler's life. Rodrigo and the others interpret this clearly as an order to kill strangers.
At the end of the journal, Rodrigo notes that one villager's head has broken off, accompanied with an illustration of a Plaga A. He sees this as a miracle performed by Lord Saddler himself, and hopes to receive more instructions from him.