The Sinner's season — and series — finale finds Harry Ambrose asleep at the wheel, literally. With the stress of Meg Muldoon's threat to his safety and the police's unwillingness to help weighing on him, he's dozed off in his car. 

But even in sleep, Harry's never far from the case. He's taking this overdue slumber at the boatyard, where he wakes to find Valerie — and all evidence within the vessel — reduced to a charred carcass. Imaginary Percy wants him to move on: "Aren't you tired? Can't we just let it all go?" But we all know Harry can't do that. 

Meanwhile, tensions are high at the Muldoon household. Meg's scolding her sons for getting involved with the smuggling ring, but Colin claims the family business would have sunk without the extra income. Sean wants to go to the police and maybe cut a deal for a reduced punishment, but the matriarch insists they stay quiet and ride out the storm.

Ambrose pays a visit to Don. He calls him out on torching the evidence and urges him to come clean. But the shady boatyard owner denies everything while his wife dials up the police. Lou Raskin shows up and handcuffs Harry. But instead of taking him into custody, he drives down the road and shares a secret: "I made a big mistake. You were right." As he removes the cuffs, Raskin explains Verne Novak did indeed kill Brandon, and officer Josh was on duty the night Percy went to him with her smuggling suspicions. But the security footage from that night is missing... and so is officer Josh.

Back at the station, Lou releases Mike Lam, who's none too happy about the chief's "lazy" police work. Meanwhile, the mainland authorities inform Harry that the FBI is already well aware of the island's human trafficking problem. The feds don't want anyone messing with their case, but the mysterious death that drove a guilt-ridden Percy to suicide still doesn't sit right with Ambrose.

The Portland PD's hands may be tied by the FBI, but Harry's got one more card to play — a local number in Novak's cell phone history. Raskin reluctantly traces its address, which leads Harry to a payphone just across from Colin's residence. Ambrose moves in for a closer look and spies a surprising group gathered around Colin's kitchen table. In addition to Meg and her boys, Mike Lam and his wife are enjoying beers and engaged in deep conversation.

This curious meeting prompts Harry and Lou to dig into the Lam's business history. On top of suddenly acquiring one of the island's coveted lobstering permits, the Lams received a large influx of cash and a tiny private island...sold to them for $10 from Sean Muldoon. Harry takes a boat out to the small, rocky property, where he discovers a gravesite adorned with a jade figurine like the one CJ gave to Percy.

Ambrose tracks down CJ, who doesn't want to discuss his family's secret island or the tiny statue.  Harry presses, asking if the item belonged to CJ's brother Bo, who supposedly lives in Hong Kong. But a series of flashbacks tells a different tale: Bo was working on the island just a couple of years prior. He was also the Lam's golden child, the mind and motivator behind his family's ambitious business plans. Harry surmises Bo is dead, buried, and memorialized on the Lam's little island. A distraught CJ doesn't confirm Harry's theory, but he still begs him to stop pursuing this line of thinking.

Under the guise of saying "goodbye" before heading home, Harry takes one more crack at the guilt-stricken Sean. He brings up the shady island sale, Bo Lam, and the sobering truth that secrets have a way of haunting you. Despite Colin and Meg's best efforts to intervene, Sean breaks down: "We killed him! Bo used to work for us, and we took him one night on our boat..."

This prompts a flashback of another Muldoon family argument. Bo had apparently been borrowing their boats without their consent to make some extra cash. So Sean, Colin, and Percy took him out one night to talk it out. What starts as a verbal argument quickly escalates to a bloody brawl between Bo and the Muldoon boys, which also sees the Celtic Star broken off the boat. Percy grabs a gun in an attempt to break up the fight, but the weapon accidentally goes off.

While Bo bleeds out, Percy trembles in disbelief over what she's done, and her uncle and father hastily address the shocking turn of events. They move the body to shore, which causes Sean to hurt his shoulder — the injury that led to his previously referenced drug abuse. They bring Bo back to their house, lay his body on the kitchen table, and make a difficult phone call to the Lams.

With Mr. and Mrs. Lam crying over the very fresh corpse of their son, Meg tries to explain the events that led to this "horrible mistake." She begs them not to call the police, as it would ruin the rest of young Percy's life. As the sun rises, the Muldoons and the Lams are yet to reach an agreement. The deeply distraught Mrs. Lam doesn't want to make a deal, but her husband convinces her a payout — in the form of the Muldoon's most valuable fishing permit — is what's best for their family and CJ's future.

the sinner
'The Sinner'
| Credit: Michael Tompkins/USA Network

Upon dropping that bomb, Sean continues to spill his guts, admitting he and Colin then got involved with the human trafficking to mitigate the financial damage done by the Lam deal. Sean wants to go to the station with Harry, but Meg begs him to reconsider: "We're going to lose everything," she pleads. He doesn't budge, tearfully replying, "It's what she wanted."

With the wheels of justice finally in motion, Harry returns to his cottage, packs his things, and dials Sonya. He gets her voicemail but doesn't leave a message. He heads to the Lam's with Lou to inform the family no charges will be brought against them for covering up Bo's murder. Harry also attempts to comfort them by mentioning Percy's extreme guilt over the accident. It turns out CJ wasn't aware of the whole truth, however, and had been told Bo was killed in an equipment accident.

Outside, Lou and Ambrose say their goodbyes and share an awkward hug. Harry's next stop on his farewell tour takes him to the church, where Meg's praying. She's bitter over his determination to uncover the truth but seems quietly appreciative when he gives her the knotted length of fishing rope that belonged to Percy.

Their brief interaction is followed by a flashback of the night officer Josh brought Percy home after she'd come to him with her suspicions. Meg walks in on her sons and granddaughter, demanding an explanation. But rather than fessing up on the smuggling ring, Colin claims Percy's still upset over accidentally killing Bo. Wracked with guilt, Percy begs Meg to let her turn herself in: "I have to go to Lou. I need to. I have to or I'm going to go crazy." Despite her desperate pleas and streaming tears, grandma makes her promise she won't go to the police. Percy looks at Sean, hoping her dad will help when she needs him most, but he remains silent. Percy storms from her home for what will ultimately be the last time.

Harry heads to the bluff to deliver his final goodbye to Percy. The two recount her fateful journey, filling in some of the blanks from earlier in the season. It turns out Brandon was the menacing, hooded figure that scared her by the docks, while Colin was the creep stalking her at her mainland retail job. But it was Bo, manifested by her guilty conscious, that she saw in the club that night and, most crucially, behind her on the bluff before she stepped to her death. 

Harry and Percy then share a tender, emotional moment, as he talks of relating to her feelings of inescapable guilt, admitting he was battling similar demons the day they met. She asks him if he now sees another way to cope. Barely holding back tears, he nods yes.

A final montage shows Colin turning himself in and joining his brother at the police station, as well as Meg seeking peace with Em at her ritual hut on the beach. We also see the Lams celebrating Bo's life at his memorial.

Before the screen goes black on The Sinner for the last time, we're given a parting shot of Ambrose. No longer helped — or haunted — by Percy, he's kneeling on the bluff in a very zen-like pose, gazing into what, we hope, is a happier, murder-mystery-free future for the beleaguered detective.

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