Upon hearing this story, Walter realizes that Fosco and Percival have deliberately switched Anne and Laura so they can get the money that would be transferred to them in the instance of Laura's death. Anxious to ensure their safety, Walter chooses a house in a poor neighborhood in East London, and finds work doing cheap illustrations. He and Marian invest the money they have in order to fund further investigations. They know it will be very difficult to prove Laura's identity, since her suffering has made her look even more like Anne. Her mind and memory are also quite unstable, so it's unreliable to ask her to prove her knowledge of only things Laura could know. They decide they will have to restore her identity without her assistance and begin by gathering as much information as they can in order to present it to Mr. Kyrle. They are disturbed when, upon asking Mrs. Vesey if Laura ever stayed with her in London, Mrs. Vesey confirms that she did not. This means that Laura's mistakes and confusion could compromise the credibility of their story. They are able to gather statements from Mrs. Michelson, the doctor, the cook, and the woman who prepared the body. Armed with this information, Walter goes to meet with the lawyer. He is anxious because he knows that Sir Percival may have returned to England upon hearing of Laura's escape, and that Percival would immediately be suspicious of him.
Walter explains everything to Kyrle, who tells him that he has no legal case. The only way there might be cause to pursue a case would be if it could be proven that there is a discrepancy between the date of death on the certificate and the date of Laura's journey to London, but neither Laura nor the housekeeper have been able to remember the exact date she left. Walter wonders if he can force Fosco and Percival to confirm this date, even though they have the most to gain by concealing it. He is determined to prove her identity. Before he leaves, Walter is given a letter to deliver to Marian, and finds out from Kyrle that Percival has returned to England. As he leaves, Walter is certain he is being watched by two men, and he makes sure to lose them before returning home.
When he returns home and delivers the letter to Marian, it turns out to be from Count Fosco. He tells her that as long as she and Laura stay hidden and secret, they will be safe. He warns her not to let Walter or anyone else persuade her to try and bring Laura's identity to light. This letter only angers them further, and Walter announces that the next day he will go to Blackwater, since he is determined to prove that she could not have died on the day stated on the certificate, since she did not leave Blackwater until after that date. He plans to try and prove this both by questioning Dr. Dawson, and by finding out what inn Percival stayed at when he left Blackwater. If neither of these strategies work, he plans to uncover the secret and then blackmail Percival into revealing the deception.
Walter goes first to Dr. Dawson, but he only has records of when he returned to treat Marian, not of how much time elapsed between then and Laura's departure. The inn where Sir Percival is known to have stayed on the night of his departure has closed down. With no other options, Walter decides to go to Blackwater Park and try to question the gardener. Neither the gardener nor the servants can shed any light on the date of Percival's departure, and Walter encounters another man who he believes is there to spy on him. Walter now turns his hopes to finding Mrs. Clements, and he sets this plan in motion by writing to Ms. Todd to ask if she has been in contact with Mrs. Clements. He also investigates Percival's background, learning that he was the only son of a baronet who made many enemies in the area around Blackwater Park, and lived mostly abroad as a result. Percival mostly grew up on the Continent; he returned to England after his father's death, and became good friends with Laura's father, Mr. Philip Fairlie, around this time. The friendship between the two men was what led to the engagement between Laura and Percival.
The answer from Mrs. Todd informs them that Mrs. Clements did write to her after Anne's disappearance, wondering if Anne might have made her way back to the neighborhood. Mrs. Clements also provided a contact address in London, and Walter immediately heads there. He introduces himself and reminds her that he helped Anne to escape from the asylum. Mrs. Clements asks if he has news about Anne, and he is careful to say that he is not hopeful about her safety, and that right now his priority is to bring to justice the men who may have harmed her. She is willing to tell him whatever she can, and Walter learns further information from her. After they fled Todd's Corner, Anne and Mrs. Clements returned to London and then from there moved to the remote town of Grimsby, where Mrs. Clements had family connections. Anne refused to return to her mother, since she feared Percival would find her there. While at Grimsby, Anne began to show signs of illness and was diagnosed with a serious heart condition.
Mrs. Clements tells Walter that when Anne learned that Percival has gotten married, she became obsessed with the idea of speaking to his wife. Mrs. Clements reluctantly agreed to go with her to Cumberland. In order to avoid being detected, they stayed at a nearby village. Anne made her illness worse by walking back and forth from the village and Blackwater Park, and eventually became confined to her bed. In order to soothe her, Mrs. Clements went to meet Laura and bring her back to the village, but she met Fosco instead. Fosco said he had a message from Laura: that Mrs. Clements and Anne should return to London, and tell her their address so that she could meet them there with greater safety. Mrs. Clements explained that Anne was too sick to travel, and that she was afraid to consult a doctor, lest it gave away their identities. Fosco offered to look at Anne himself, and he and Mrs. Clements returned to the cottage, where he prepared some medication for Anne. This helped her to regain some strength, and within a week Fosco had accompanied them to the train station and helped them return to London. Once there, Mrs. Clements wrote to Laura with their address, as Fosco had instructed. About two weeks later, while Mrs. Clements was distracted, Anne vanished from the house. Mrs. Clements looked for her at the asylum, and at the home of Mrs. Catherick, but gave up when Anne could not be located at either place.
Once Laura, Marian and Walter are safe in London, they are faced with a strange dilemma. So long as they keep their location secret, there is no imminent threat to them, and as the letter from Fosco seems to indicate, as long as they don't try to bring the scheme to light, Fosco has no interest in tracking them down. While it would require them to live in poverty, there's no real reason why they can't simply continue in their current situation indefinitely. In fact, this new set up offers some advantages in that all three of them get along well, and Marian and Laura can now actually enjoy more independence than they did while living as upper-class ladies. It is therefore somewhat striking that Walter becomes so obsessed with re-establishing Laura's identity. While it is easy to see why he would want justice served, and to know that Fosco and Percival did not get away with their crime, there's no evidence that he ever asks Laura what she wants, and whether it is important for her to have her identity re-established.
This section, in fact, makes it clear that Marian and Walter are making all the decisions and essentially treating Laura as a child. To be fair, the impact of her ordeal does seem to have left her mentally unbalanced, and she cannot advocate for herself at this point. While on one hand confirming Laura's feminine fragility and suggesting that she is not intellectually capable of the clear memory and accuracy that allow Marian and Walter to serve as good detectives, this response works to unsettle the dichotomy between madness and sanity. As both Anne and Walter have already demonstrated, a perfectly sane person can show signs of becoming unhinged if subjected to enough trauma. The irony that it is Laura's being declared mad that makes her start to actually lose her grip on her mental faculties raises some troubling questions about other individuals who may be wrongfully committed but then over time come to be, to all appearances, "actually" mad.
By now, Marian and Laura have repeatedly been let down by structures and individuals that seemed designed to help them, and now it is Walter's turn for this experience. He assumes that because a crime has been committed, the first step is to appeal for legal assistance, and he is horrified to learn that the law will not be able to help him. The limitation of the legal system is its reliance on clearly documented evidence, and as the novel has shown over and over, evidence can be missing, inadequate, or deliberately misleading. Walter and Marian know the truth, but need to find some way to support it with evidence, otherwise it can be dismissed as simply a story. Walter will have to function as a kind of independent investigator.
Walter's initial investigation does shed further light on the active role Fosco played in the plot, and in his charismatic and manipulative ability to get people to trust him. Mrs. Clements made it very easy for him to gain access to Anne, which led to him abducting her. While Mrs. Clements is far more benevolent and well-intentioned than many other characters, she has also not been able to protect Anne despite loving her like a daughter.