The Sinner's fourth episode wastes no time addressing last week's floating-corpse cliffhanger. The body belongs to Percy Muldoon. Turns out Detective Harry Ambrose's eyes did not deceive him that fateful night on the bluff — the troubled girl did indeed step off the cliff to her death.

In fact, Chief Lou Raskin — somberly echoing the coroner's findings to the Muldoons — confirms it: "He didn't find evidence of foul play... she died from the fall." Still, upon delivering the news to Meg, Sean, and Colin, he offers to keep the investigation open. But Uncle Colin quickly squashes the idea. He wants to bury his niece and move on.

Of course, none of this sits well with Harry, who's just discovered a Celtic North Star totem — with the name "Valerie" crudely etched into it — among the items found on Percy. He heads back to the cottage and tells Sonya they're closing the case. He feels there are still lots of unanswered questions, but she understands the Muldoon's decision to put the pain behind them. She wants to leave the island. Harry reluctantly agrees.

But first, he brings the totem to the docks, where he finds others just like it on Colin's boat. He speaks to a Muldoon employee, who tells him Colin and Percy used to take the vessel out on non-fishing trips. He tracks Colin to an AA meeting, where he overhears him unload a greatest hits compilation of incriminating statements. He talks of how much he loved Percy but also references a "big fight" between them and "something he did" right before her death.

Harry's packing his bags when a call comes in from Meg. They meet at a church, where grandma Muldoon gets right to the point: "Percy didn't do this to herself. Someone's to blame for this." She also informs him that Percy was devout, but someone turned her from her faith and had control over her. More than ever, she wants Harry's help.

Ambrose has a final lunch with Lou at the Lam's restaurant, but there's more than clam chowder on the menu. He speaks with CJ and his dad about their ongoing rivalry with the Muldoons. Like the dock worker Harry questioned, Lam Sr. talks of Colin and Percy taking his boat out. "There's something wrong with him and her," he says. A flashback of the pair engaged in what looks like a lover's quarrel confirms this.

Back at the soon-to-be-vacated cottage, Sonya enthusiastically arrives with scallops and shrimp to prepare for their final meal on the island. But instead of a hungry Harry, she finds a ransacked room, muddy footprints, and a red raincoat-wearing intruder lurking inside. When Ambrose returns, she tells him she didn't stick around to identify the trespasser and, understandably, she wants to leave first thing in the morning.

Before boarding the ferry the next day, Harry asks Sonya for more time to work the case. She's not having it. "I don't think I can keep going on like this," she says before heading to the boat by herself. Ambrose can't bring himself to join her, despite imaginary Percy telling him to do so. Instead, he heads to the funeral, where Colin and Percy's ex, Brandon, are serving as pallbearers. He then attends a memorial service at the Muldoon's, where pictures celebrating Percy's life show her and Colin looking close, figuratively and literally. Across the room, Colin angrily grabs Brandon by the collar.

Harry talks to Sean in the kitchen. The grieving father is feeling guilty, suggesting everyone turned a blind eye to the destructive path that led to Percy's death. Brandon, in a red raincoat, comes in and gives Sean a warm hug. Harry follows him but is stopped by Meg. Harry tells her about the intruder, hinting Colin could be behind the break-in and Percy's demise. He treads carefully but brings up their relationship. Meg doesn't like the implication. She admits her son and granddaughter were very close but insists they shared a father-daughter bond.

Harry caps the evening with a visit to Colin's boat. While searching the cabin, someone locks the door, and the engine starts up. As the vessel heads out, the door mysteriously unlocks. Harry emerges from the cabin to find a smug Colin: "So you're searching my boat now?" he asks. Despite the dire turn of events, Ambrose keeps his cool: "Trying to get the whole picture," he replies.

Defensive as always, Colin asks why Harry's been inquiring about Valerie (the name etched into the Celtic totem found on Percy). Harry points to the similar totems on his boat before prying into Colin and Percy's seemingly inappropriate relationship. He lays his cards on the table — "I think you loved Percy... maybe a little too much." Muldoon's infuriated by the suggestion of an illicit romance between them.

Things take a dramatic, unexpected turn — for both Harry and viewers — when Colin starts talking about God, faith, and his efforts to help his niece connect with a higher power. A flashback shows the pair praying together on his boat, but Percy's not into it. As Colin earnestly puts his hand over her heart and speaks of her broken relationship with God, she seems frustrated and distant.

Colin tells Harry, "I thought I could reach her, but someone else had gotten their first." Another flashback sees a rain-drenched Percy showing up at Colin's doorstep, asking to stay the night. He wraps her in a towel, and it's clear he cares for her in a non-creepy, caring-uncle way. But he's still a jerk. In another flashback, he confronts Percy after she misses a bible-boat meeting. He grabs her and calls her an "ungrateful f---ing  bitch." "I raised you," he says. To which she angrily responds, "No one asked you to. You're not my f---ing dad." This bombshell revelation not only paints a very different picture of their "relationship" but also explains that "big fight" Colin expressed guilt over at the AA meeting.

Back on the boat, Colin tells Harry, "I could have gone easier on her," and we actually feel a bit sorry for Uncle Colin. He brings Ambrose a folder of unusual artwork — including a picture of a snake surrounded by various letters and eating its own tail — drawn by Percy. Colin says someone poisoned her mind, but Ambrose isn't letting him dodge all the blame: "You filled her head with what you wanted, and you couldn't stand to share her with anyone else." The assertion gets Ambrose pushed in the water. Harry makes it home (it's not entirely clear if he swam to shore or Muldoon felt bad and scooped him up) and immediately takes a hot shower to warm up. He then tries to recreate Percy's drawing from memory, scribbling a snake and attempting to recall the letters surrounding the serpent.

Imaginary Percy helps fill in some blanks, but it's an earlier encounter with Sonya that provides a light-bulb moment. On their initial trip to Hanover Island, Ambrose's now-ex reads about their destination's history, including references to the "Bazegw Legend." This sends Ambrose to Bazegw Point, where he discovers several displays of random objects – similar to Percy's assortment of trinkets found on the bluff – stretched before him on the beach. He follows them until he arrives at a ceremonial hut made of sticks. Inside he finds feathers, candles, small animal skulls, and a sculpture of a snake consuming its tail.

But the most startling discovery comes just before the credits roll. Harry spots a scroll of paper, which he unfurls to reveal a disturbing bit of artwork — a monstrous figure with red, slit eyes is embracing a young woman. The drawing's reverse side shows the artist's signature: "P. Muldoon."

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