Walter returns to London, having learned that Marian and Laura were obliged to move houses in his absence. He decides not to reveal what he has learned about Sir Percival, since the rightful heir has now come into the property anyways, and none of this helps with reestablishing Laura's identity. He is dismayed to learn from Marian that while he was away, Marian was visited by Count Fosco. She managed to conceal the visit from Laura. Fosco arrived with the owner of the asylum, and summoned Marian to meet with him. He explained that he had known where they were living for awhile, but had no interest in persecuting them so long as all secrets remained concealed. However, now that Sir Percival is dead, Fosco has become concerned that Walter will start investigating him. To prevent this, he brought the asylum owner to the house, planning to reveal Laura as his runaway patient, and assuming that the results would occupy Walter and leave him no time to investigate Fosco. However, at the last minute, he aborted the plan because of his admiration for Marian and his worries of how devastated she would be to lose her sister. Fosco does warn Marian that Walter should be very careful about deciding whether he wants to continue his investigations, because he, Fosco, will stop at nothing to protect himself.
Marian decided they had to move, and without telling Laura why, found them new lodgings. Walter reassures her that he doesn't think Fosco would be able to do much about putting Laura back in the asylum at this point, and also proclaims that he will not be frightened away, and will persist in championing Laura's cause. They break the news of Sir Percival's death to Laura, and settle down to wait for the moment to be right to continue with their plans. Walter has evidence that Fosco is not planning to leave England any time soon, so he knows that he has time. He also digs deeper into the mystery of Anne's father and uncovers the truth. During the time that Mrs. Catherick was working as a maid, Mr. Philip Fairlie (Laura's father, and a notorious womanizer) regularly visited the house where she worked. He was Anne's father, and this explains the striking resemblance between Anne and Laura.
A few months later, with Laura growing stronger, Walter asks Marian about the possibility of proposing to Laura. He suggests that being his wife will give her more legal protections, and make him better able to advocate for her. Marian readily agrees, and he and Laura marry only days later. Although his marriage makes Walter more conscious of the risks he is taking, he is still determined to bring Fosco to justice, and begins investigating him. Based on what he knows of the Count's skills and correspondence, he begins to believe that Fosco is a spy. In order to understand more, he reaches out to his Italian friend, Professor Pesca. After following Fosco one afternoon, and determining that he plans to attend the opera that evening, Walter and Pesca secure tickets to the same performance.
While there, Walter sees the two Italians look at each other, and realizes that the sight of Pesca terrifies Fosco. After the performance, he questions Pesca asking if he knows of any reason why Fosco would be afraid of him. Pesca reluctantly confides that he is a member of a secret Italian political society; he moved to England at their request, and awaits orders to return at any moment. The members do not know the identities of any other members, and are sworn to secrecy at the expense of their life. They are all branded with a secret mark. If someone betrays the brotherhood, he is eventually hunted down and killed. From this explanation and Fosco's reaction, Walter infers that he has betrayed the society and believed that Pesca may have been sent to kill him.
Walter realizes that Fosco is probably going to flee London as soon as possible. He prepares a letter for Pesca to read in the event of his death, and then goes to Fosco's house that very night. He confronts Fosco, who is on the verge of shooting him, but warns that if he is harmed, Pesca will come after him. Fosco reluctantly agrees to listen to Walter's demands: Walter asks for a signed confession, and proof of the date on which Laura departed from Blackwater Park. Fosco says he will provide both those things, so long as Walter allows him and the Countess to leave London and does not pursue them. Walter hesitates, but then agrees to these conditions. Under Walter's supervision, the Count writes his confession, produces a letter from Sir Percival that explicitly refers to the date of Laura's departure, and then leaves with the Countess.
With Percival dead, Fosco is the only threat that remains, and he seems willing to maintain a truce. He has no particular interest in persecuting the trio, although the visit makes it chillingly clear that he could very easily harm them in some way if he wanted to. His bizarre declaration of affection for Marian also helps to explain why he would be happy to leave things at the status quo. Especially now that Percival is dead, and Walter and Laura can marry, there is an even stronger case for not pursuing the investigation and antagonizing Fosco. Yet Walter remains stubbornly determined to see Laura's identity restored, even though he knows that Fosco is an extremely dangerous enemy.
The death of Percival is presented as less significant for ensuring Laura's safety than for ending her bond with a man she despised. Given that it would not have been possible for her to annul or divorce, Percival's death frees her to marry a man she actually loves and someday have a family. Walter and Laura's marriage is presented somewhat strangely. Ever since being reunited with her, Walter has carefully maintained a brotherly relationship with her, but now that she is widowed his attitude towards her shifts rapidly. He seeks out Marian's permission, acknowledging Marian's role as a sort of guardian to her sister, and also that he views Marian as a kind of masculine equal. At the same time, he presents the marriage as being rooted in his desire to have the legal status as Laura's guardian and protector, not his obvious love for her. Thus, even in her second marriage, terms and conditions are negotiated without Laura's consent and part of the motive is control and power.
Walter's marriage makes him even more determined to secure Laura's identity and he is prepared to risk his life by confronting Fosco. Once again, a written document, this time in the form of a warning to Pesca to initiate pursuit, safeguards Walter and allows him to reach an agreement. True to form, Fosco carefully negotiates an agreement that ensures his protection. The evidence necessary to verify who Laura truly is relies on written statements and documentation; these function as the form of testimony and evidence that will be reliable enough to withstand legal scrutiny. Yet given how many times documents have been proven false and unreliable, it seems ironic that it is finally documents that bring the entire ordeal to a close.
Fosco's participation in the secret Italian brotherhood complements his exoticized identity and suggests that his weakness has been his belief that he could outwit everyone. He has not taken seriously what it means to betray the brotherhood, and the very organization that taught him how to be an adept and slippery spy and manipulator finally outwits him in the end.