All the family secrets come spilling out in The Woman In White's final episode.
Marian: If either of those men are at your mercy, and you are obliged to spare one of them, don't let it be the count.
Last week ended with Marian advertising all over town that Laura is alive and well and no longer living in an asylum. This week picks up with the immediate repercussions for Glyde and Fosco. Percival, it turns out, is no good at this lying game, panicking that his wife is talking to the police. Fosco is far more sanguine. No one believes women in this day and age. Interestingly, Nash is outside the Fosco residents snooping and hears Fosco telling Glyde to chill out and stick to the plan.
Laura is being held in a rental house, with Marian and Walter, where she has mentally collapsed from the stress of her experience and grief over learning Anne is dead. She's suffering from PTSD and struggling with suicidal thoughts, neither of which are conditions that anyone knows how to deal with in this time period. Marian seems confused at Laura flinching away from her. Walter, on the other hand, just puts himself on suicide watch and occasionally wrestles his charge off the roof.
Walter wants to track down Glyde's secret, but Marian is more concerned about establishing that Laura is who she says she is. Walter thinks if he asks Anne's mother about Glyde, and to identify the woman they have in the bed, perhaps they can kill two birds with one stone. Meanwhile, Fosco is still trying to communicate with Marian and has at least figured out Kyrle knows where she is and will get threatening letters to her. But Marian has woken up to Fosco being the real danger, even if she doesn't yet know how weak Glyde really is.
Walter, meanwhile, is trying to get Nash to show them a path forward, but instead, Nash continues to point out they don't have a case they can win yet. (Walter is foolishly unappreciative of the service Nash is doling out by not letting them run off half-cocked.) Nash admits he has not spoken to Mrs. Catherick. Walter is going to see Anne's mother. The problem is, Glyde is having Mrs. Catherick (Kerry Fox) watched, for this very reason. Unaware of this, Walter goes to see her anyway and finds a woman who is far more difficult than one might assume. Learning Anne is dead, Mrs. Catherick declares herself in mourning, while sneering Walter knows nothing of Percival's secrets or family. But when Walter pushes, suggesting he knows the secret of Anne's father, she throws him out.
Walter: Do you have a single human bone in your body?
Nash: I did, a few. I found finally they were redundant and quite possibly a hindrance.
Marian and Walter grab onto Mrs. Catherick's hint about Glyde's family, with Marian deciding to put some research into discovering who Glyde's mother was. Turns out the marriage took place in Welmingham, the same place Mrs. Catherick lives, sending Walter back on the train to where he just came from, though this time to the church not far from her house. After fumbling with a very cranky lock, the parish clerk gets in to find the marriage records. The marriage entry is there, but it looks irregular. Since it's a copy, the clerk gets the original registry to compare. There's no entry with Glyde's marriage. It does not exist. Percival had the entry forged to hide the fact that he was born on the wrong side of the bed.
When Glyde gets word that Hartright showed up at the Catherick house, he freaks. Drunk and hysterical, Glyde races out, much to Fosco's disgust. Meanwhile, Walter races off to tell his lawyer, leaving the evidence behind, just as Glyde's ride pulls into Welmingham, heading for the church. Glyde breaks into the church, but realizing he's caught when Walter returns, he sets the registry on fire as a distraction. But the lock traps him inside. By the time Walter and company break down the door, he is dead. The evidence may be gone, but so is Laura's very inconvenient husband, and his claims of Laura as Anne.
Laura, meanwhile, is practicing the 1850s version of music and art therapy and slowly coming back to herself, using Marian as talk therapy. When Fosco shows up, to cover his own part in this mess, Marian refuses, and Nash turns up right on time to keep her from being threatened. Nash is there to tell Laura about Glyde's death. When Laura says she forgives Glyde and hopes he finds peace, Nash is moved by her heart.
Walter doesn't come back right away, because he's put a few things together. The old parish clerk was Mr. Catherick. His wife was the one who let Percival tamper with the marriage registry. She says she didn't have a choice. Glyde was blackmailing her because he knew her secret: Anne's father was Philip Fairlie, Laura's father. (I believe we called this twist back in Episode 3.) The reason Fairlie took it into his head to engage Laura to Glyde was also blackmail on Glyde's part. Turns out Lady Fosco was onto something about Laura being a pawn.
Marian: I will never, ever bargain with you.
Anne's mother has no interest in helping Laura, she's too bitter. Laura declares that she wishes to return to Limmerage again. But this is impossible since they still can't prove she's not Anne. Marian thinks the whole lot of them should just move, but Laura is dead set against running anymore. Walter and Nash decide the best answer is to figure out Fosco. Walter suddenly remembers Mr. Pesca (remember him from episode 1?) is also Sicilian, and politically active in his home country. He could have some useful dirt. Nash keeps an eye on Fosco.
The two of them engineer to have Pesca and Fosco see each other across the room both of whom then freeze and flee. Walter has to force Pesca into admitting he knew Fosco, both were members of the Blood Oath Society, a 1850s people's uprising against tyranny and oligarchs. Fosco betrayed them. Pesca agrees to be Walter's heavy in a ploy to force a confession. Fosco's response is to pull out a pistol, which his wife coldly advises him to use. Walter insists Fosco will be spared Pesca's wrath if he confesses to Nash what was done. The Foscos will then be allowed to leave the country. Nash coolly enters to take the confession. Fosco insists Anne was not murdered, she died of natural causes, as she told Laura she was. As far as Fosco is concerned, no murder, no crime. That the lives of women were ruined (and Glyde's) is not his concern. Identities are mistaken, but Laura's alive, so no harm, no foul.
Walter has his confession and his proof. Pesca takes Fosco's life anyway. Uncle Fairlie is forced to allow Laura back and agrees to bless her marriage to Walter (by his own butler, no less.) Marian goes out to travel the world. Anne gets a proper headstone. And with that, they all live happily ever after.