After leaving Mrs. Catherick's house, Walter realizes he has been followed by one of the spies, planted there to wait for him to come to question Anne's mother. Nonetheless, he decides to continue with his plan of visiting the church, especially since he is intrigued by a sarcastic comment Mrs. Catherick made about Sir Percival's mother. Walter intends to look at look at the marriage register to see what her name and family background were. He perseveres in doing so, even though he suspects that more spies observe his approach to the church, and likely convey the progress of his investigation to Sir Percival. However, the visit to the registry does not at first seem to be fruitful. Walter is able to find an unremarkable record of the marriage of Sir Felix Glyde to a Cecilia Elster a year before Sir Percival's birth.
Walter sets off for another meeting he hopes will reveal information, but along the walk he gets into a minor scuffle with the two men who have been following him. He is arrested and will be held for three days; Walter knows that this gives Percival time to cover up whatever information he might be getting closer to finding. The one person in the area who might be able to vouch for him is Dr. Dawson, to whom he has been introduced by Marian. The Doctor posts his bail, and Walter is freed, giving him a few precious hours in which Sir Percival will not be watching for him. Walter goes to inspect the duplicate copy of the marriage register, held off site. He is astonished to discover that this version contains no record of the marriage of Sir Percival's parents, revealing that Sir Percival must have forged the entry in the church copy so as to disguise his illegitimacy.
Walter knows that he will need both copies for comparison to prove the forgery, so he is in a haste to get back to the church. He hurries along the road, evades another attack, and arrives to find the church clerk distressed because he cannot locate the keys to the vestry. The clerk and Walter hurry to the church, having been alerted by a local boy that someone seems to be inside the church and preparing to start a fire. When they get closer, they see that the church is already on fire and that Sir Percival has gotten trapped inside due to the faulty lock. Walter organizes some desperate efforts to try and save him, but Sir Percival dies in the fire.
Reassured that Laura and Marian are safe in London, Walter stays on to be questioned about the strange events. He does not reveal his own knowledge about why Sir Percival was in the church, but offers his assumed timeline to readers. Walter suspects that when Sir Percival learned that Walter was free on bail, he became desperate. He ordered the attack, and also stole the keys and snuck in to the church in order to remove the page with the forgery so that Walter would have no proof. Once in the office, he locked the door in case anyone else tried to come in, not knowing about the faulty lock, and then accidentally started a fire with the lantern he was using.
While waiting around as part of the investigation, Walter receives a letter from Mrs. Catherick, thanking him for having pushed Sir Percival to the actions that led to his death. She explains that when she met Sir Percival while pregnant with Anne he bribed her with gifts and flattery into giving him access to the church registry office. Unbeknownst to Sir Percival, she spied to see what he did with this access, and once she found out that he had committed forgery, she used this information as blackmail for more presents. Sir Percival also gave her more details about why he is committing the forgery. His parents could not marry because his mother was legally married already, but had run away when her first husband abused her. While Percival was a child abroad, everyone assumed his parents were married, and when his father died, there was no reason anyone would be suspicious when Percival returned and claimed his estate. However, in order to borrow money, additional documents (including the marriage certificate) were required, and this would have to be substantiated with a church record of the marriage. Thus, after obtaining access to the church office, Sir Percival inserted the record of his parents' marriage at an appropriate date.
Mrs. Catherick initially did not think Sir Percival was doing anything terribly wrong, and was happy with the gifts. When she realized her husband believed they were having an affair, however, she asked Percival to save her reputation by telling him they were not. Percival refused, because the idea of an affair made it less likely anyone would ever uncover the real secret, and this scandal wouldn't really affect him, even though it would ruin her. Mrs. Catherick became very angry and threatened to expose him, but Sir Percival revealed that she was now an accomplice to his crime, and that if he was prosecuted, she would be as well. He is, however, willing to provide her with an income as long as she keeps the secret, and stays in the town, where he can keep an eye on her. Abandoned by her husband and feeling she has no other options, Mrs. Catherick agreed.
Mrs. Catherick goes on to explain that she never liked her daughter, but found the presence of the child useful in gaining public sympathy and gradually rebuilding her reputation. One day, after the time at Limmeridge, Mrs. Catherick was frustrated with Sir Percival having refused a request and angrily alluded to the fact that she could ruin him, without realizing that Anne had overheard. A few days later when Percival came to visit, he made Anne angry and she repeated the comment about ruining him, making it seem as though she also knew the secret. Sir Percival flew into a panic at the idea that Anne might know his secret, since he could not control her the way he controls her mother, and insisted on sending her to the asylum. Mrs. Catherick did not particularly object, since it would make her life easier; thus, the two of them had Anne declared insane and sent away. Since then, Mrs. Catherick has never been too worried, since despite Anne liking to refer to the secret, she has never known the truth about Sir Percival's illegitimacy. She also warns Walter not to continue investigating Anne's paternity, which leads him to believe there is still a further secret remaining there.
After such a long build-up, Sir Percival's secret is somewhat surprising because of how relatively benign it is. It is certainly not his fault that he was born illegitimately, and while the forgery was a very serious crime, it didn't actually hurt anyone. Walter's own decision not to publicize his knowledge of why Sir Percival was in the church, as well as his efforts to save Sir Percival's life, suggest that he is also somewhat unmoved by what he finally discovered about Sir Percival's past. The nature of his crime is interesting given the novel's thematic preoccupation with documents and written records. As has been seen in other cases, Sir Percival changes a written record to reflect a falsehood, assuming that no one will question the authority of the document. In fact, the plan works so well that Walter is not even suspicious when he first sees the marriage record. The tampering with the document also raises questions about legality and the reliance on law. The record being included or not does not change the relationship between Sir Percival's parents, or his genetic heritage. The crime reveals the fragility of the law, and the fact that the entire course of someone's life depends on a simple notation.
Despite these anti-climatic elements, Sir Percival has engaged in many villainous activities to cover up his crime, and his death offers a convenient kind of poetic justice. He is desperately trying to erase evidence of his crime and ends up causing his own death. This satisfies a reader's desire to see the villain punished, but it conveniently prevents Walter from having to kill him directly. In fact, Walter's heroic attempts to try and save Sir Percival's life further vindicate his integrity and sense of honor. Readers can both see the bad character punished, but also know that Walter remains a good and honorable man.
The anti-climatic effect is furthered by the confirmation of Walter's suspicion: Anne Catherick never actually knew Percival's secret. In a sense, this makes most of the plot a tragic mistake that could have all been avoided with greater knowledge. Anne was actually powerless, but Percival's anxiety that she might be able to undermine him led him to destroy her. Because he knew he was living a lie, and that everything he had could be taken away at any moment, he became increasingly paranoid and obsessed with controlling anyone who might have knowledge of his true history.
The admission that Percival and Mrs. Catherick conspired to have Anne put in an asylum when they knew she was sane represents chilling confirmation of how individuals, especially women, who are perceived as burdens or threats could be cast away in Victorian England. It also makes it clear that the protections supposedly in place for vulnerable people like both Anne and Laura was inadequate. If the individuals who were supposed to care for them failed to do so, institutional structures were not going to safeguard their welfare.