When we last saw Harry Ambrose (Bill Pullman), he was coping in the aftermath of the most harrowing case of his 40-year career. The Sinner's brilliant, perpetually overburdened detective had been beaten, shot, and buried alive. But that wasn't the worst of it. Having killed last season's sociopath antagonist Jamie Burns, Harry was wracked with guilt and bordering on a mental breakdown.

But things are looking up at the start of season 4. The one good thing to come from the Burns' case (Harry's romance with Sonya) appears to be going well, as the couple's on a ferry set for a lovely fishing town in Maine. Still, despite the promise of some much-needed R&R and lobster rolls, recently retired Harry seems a bit distant and troubled.

They arrive on Hanover Island to meet Greta, a self-described "broke, lesbian painter" and friend of Sonya's, who sets them up in a quaint cottage by the water. Sonya (Jessica Hecht) suggests the sound of the waves will help Harry with his insomnia — another sign all's not right in his head.

At lunch, Ambrose and Sonya awkwardly answer Greta's questions about Harry's retirement and how they're holding up after the Burns' ordeal. Harry excuses himself, leaving his hardly-touched lobster roll behind, and goes for a stroll. He soon meets the amicable Percy Muldoon, a "girl fisherman," who works for her family's company. She cryptically alludes to her connection with mother nature and mentions the ocean's "in a mood today."

Later that night, it's revealed Harry's been taking antidepressants. He's intentionally, and secretly, left the pills behind though, eliciting some concern from Sonya. While taking a walk — damn you, insomnia — he's flashing back to the night he shot Jamie when, suddenly, he hears someone scream, "Leave me alone. Stop it!" It's Percy. He follows her into the woods, where he hears exasperated, possibly pained cries. He spots Percy on a cliff's edge, peering mysteriously over her shoulder. As Harry looks on, she calmly steps off the bluff.

The police arrive, led by the well-intentioned, but clueless chief Lou Raskin. They can't find a body, so he questions if Harry's been drinking or using drugs. Muldoon matriarch, and Percy's grandmother, Meg arrives. She's upset, rude, and ultimately dismisses Harry's claims. Percy's uncle Colin is also there, and also not happy. He asks Harry, in an accusatory tone, how he knows Percy and why he followed her. While Raskin sheepishly attempts to calm the Muldoon clan, Meg continues to berate Harry.

The next morning, Harry recounts the previous night's events to Sonya. He's doubting himself, wondering if his eyes had deceived him. He puts a call in to Raskin, who informs him Percy was seen driving her car 30 minutes after Harry saw her fall/jump. Harry returns to the scene, steps onto the exact spot Percy had been, then looks over his shoulder just as she had. This leads him to a small patch of dirt hosting an odd assortment of trinkets, including a crucifix and a small piece of rope knotted into a curious design.

He brings the items to the chief, who doesn't have the slightest idea what to do with them. Harry suggests running fingerprints, to start. Raskin has run a background check on Harry and is quite impressed by his track record. Harry leaves, somewhat reluctantly, as it's clear he'd rather work the case than go see the tourist sights Raskin's recommended.

Ambrose goes with his gut, heads to Muldoon Fish Co., and finds Percy's father. While questioning him about his daughter's activities, Meg arrives. She quickly sends Harry packing, before starting an argument with a man outside. A nearby fisherman informs Harry the target of Meg's f-bombs is Mike Lam, one half of a married Asian couple that runs a rival fishing operation. The fisherman also tells Harry he saw Percy at a local watering hole, The Knot, before she disappeared. Harry heads to the pub, where the bartender tells him Percy's "exactly like her grandmother, a boss."

The Sinner
Bill Pullman in 'The Sinner'
| Credit: Michael Thompson/USA Network

While questioning some of the patrons, Ambrose learns Percy was in great spirits that night, even performing a song with a friend on stage. This is recounted in a brief flashback, which also shows Percy exiting the restroom with blood dripping from her head. When questioned by her friend, she dismisses the wound claiming she injured herself on the paper towel dispenser. 

Fast-forward to present-day, 2:27 a.m. Harry's insomnia sends him outside to repair a broken gas grill. While playing handyman, he hears an odd series of cries and grunts in the distance. The next morning, he meets Raskin, who's nervous about "screwing up." He's found Percy's car, key still in the ignition, and doesn't want to upset the apparently powerful Muldoon family. Harry and Lou question a nearby resident in an RV. Sporting a neck tattoo and plenty of attitude, the potential witness offers no help before slamming his door on them.

Over breakfast, Harry and Sonya discuss the pros and cons of him helping with the case. He's clearly already deeply invested, and Sonya's cautiously supportive. The meal's cut short by a call from Raskin. The chief and Ambrose head to the Muldoon residence, as Meg's apparently now very keen on having Harry help find her granddaughter. Three days in, she's had a change of heart — Percy's still missing, Lou "doesn't know his ass from his elbow," and leveraging Harry's seasoned detective skills is starting to sound like a good idea.

After apologizing and pleading for his help, Meg offers some backstory on Percy. It turns out her mom abandoned her at a young age, so Meg raised her as her own. Despite them being close, Meg feels Percy was hiding something from her. Apparently, she'd been uncharacteristically keeping late hours, reading a book called The Lunar Goddess, and collecting "weird" trinkets — including one that looks similar to the rope Harry found at the scene. This cues a flashback of Meg and Percy sharing a tender moment; the former tosses out some fishing rope, but the latter uses it to "preserve the moment" by tying it in a knot and "giving it meaning." New, nice Meg lets Harry take the book, and tells him he's got an open invite for a home-cooked meal.

Harry heads back to the spot he first met Percy, Pier 2. He finds a discarded pack of what appears to be nicotine gum and notices a nearby security camera. He tracks down the footage, which includes a creepy clip of Percy assuming a kneeling/worshiping position in front of a hooded figure. Scared, she slowly backs away from the mystery person before taking off. Harry's convinced someone "has a hold on her," and that same someone was at the bluff the night she disappeared.

Another sleepless night finds Harry tinkering with the gas grill and, again, hearing bizarre sounds from the woods. This time he follows the chant-like cries until he comes across a group of nude women performing some sort of ritual beneath the night sky. Ambrose watches from a distance until one of them suddenly glances in his direction. It's difficult to make her out in the darkness, but she looks a good bit like Meg, only with her hair down and, ya know, making worshiping gestures toward the stars. 

Distressed and sleepy, Harry returns to the cottage. He sits on the couch, as a female voice says, "Whatever you think it is, you're probably wrong." Harry agrees, "I know." "I could be anywhere.," the voice continues. With Harry's fragile psyche apparently playing tricks on him, the camera pans to Percy, who's sitting across from him. She concludes their haunting exchange with, "Please... find me."

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