Travel east of London toward River Blackwater and you'll find Osea Island, a tiny spot that's, in the past century, housed a military base, a rehab facility, and a recording studio favored by musicians trying to get away from it all. Or, arrive at the wrong time, and you might just find water. Only a narrow causeway connects Osea to the mainland, and then for just two four-hour stretches each day when the tide is out. Get stuck on the wrong side and you have to bide your time until nature clears the way.

Not that Osea seems like a bad place to get stuck — at least the one that exists in the real world. The Osea of The Third Day, well, that's a different story. Ask the locals and they'll tell you its Celtic inhabitants regarded the island as the secret heart of the world, and that if Osea fell out of balance, the ill effects would be felt all over. Keep asking and you'll find that belief hasn't entirely faded over the years. That's a lot of responsibility for the small community of locals to bear, particularly one in the grips of financial struggles that have forced it, with some reluctance, to open its annual summer festival to the outside world so everyone can share in the spectacle of men in fish masks jokingly pursuing naughty children with scissors and unnerving religious iconography that freely mixes Christian images with pagan traditions. This Osea Island buzzes with strangeness and practically begs casual visitors to flee lest they get swept up in its mysteries. If, that is, they can.

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That's the Osea that greets Sam (Jude Law) when he makes an unplanned visit to the island after rescuing the teenaged Epona (Jessie Ross) following her attempt to hang herself in the woods. Taking her back to her island home, Sam plans only to drop her off, see that she's okay, then be on his way. Osea, however, has other plans. After lingering in a pub owned by the chatty Mr. Martin (Paddy Considine) and his sour, sharp-tongued wife Mrs. Martin (Emily Watson) and witnessing some strange events outside its window, he can't help but feel he has more to do. When he misses his chances to return to the mainland it comes almost as a relief. Besides, if nothing else, he has to figure out what's going on with the little boy he keeps seeing everywhere, a kid with a tendency to disappear when pursued. There are lots of places to disappear on Osea.

Jude Law, <em>The Third Day</em>Jude Law, The Third Day

An unusually structured limited series that will air near-simultaneously on HBO and Britain's Sky Atlantic, The Third Day is the co-creation of writer Dennis Kelly (Pulling) and Felix Barrett. Barrett is the founder of Punchdrunk, a theater company famed for one-of-a-kind, site-specific productions like Sleep No More, in which audience members wander through, and sometimes interact with, a noir-inspired reworking of Macbeth staged in a five-story hotel. HBO will air six episodes of The Third Day over six weeks. The first three comprise the "Summer" chapter, written by Kelly and directed by Marc Munden (The Secret Garden). The second three make up "Winter," co-written by Kelly, Kit de Waal, and Dean O'Loughlin, directed by Philippa Lowthorpe (The Crowne), and starring Naomie Harris as Helen, a woman who visits Osea several seasons after Sam's stay accompanied by her daughters. Between comes "Autumn," a live, continuous-take broadcast featuring Law and other cast members that promises to reveal "the rituals and traditions of the islanders." This will only air once on October 3rd in the U.K. (meaning truly dedicated North American viewers will have to plan on watching it at odd hours, though HBO says missing it will have no impact on the enjoyment of the TV series).

The series' origins help explain some of its unusual qualities. It works well as an unfolding mystery story (at least in the five episodes provided to critics) but places as much emphasis on the world of Osea, from the strange knick-knacks and news clippings on display in the Martins' pub to a religious meeting Sam observes but whose purpose he never quite figures out. Both "Summer" and "Winter" rarely leave their protagonists' side, putting us in their place as they wander their often bewildering new surroundings and accumulate clues as to what's going on, not unlike attendees at an immersive theatrical experience. While plenty of series can feel a bit like puzzles that ask for viewers to untangle them, few take that quality quite as far as The Third Day (and the ways in which "Winter" echoes "Summer" play this up even further).

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It might be frustrating if The Third Day wasn't so spookily compelling, or didn't keep offering up new wrinkles and unexpected twists. By the end of the first episode, Sam has revealed himself to be something of an unreliable narrator as details of a past family tragedy and some more recent drama come to light. Further complicating things: a new friendship with Jess (Katherine Waterston), an American academic studying Osea's traditions with a mix of fascination and amusement that puts Sam at ease. By the end of the third, that ease has disappeared. The Third Day starts to blur the line between fantasy and reality as threats that have simmered since Sam's arrival reach a boil and scene after scene emphasizes the heat of the summer and the striking colors of the verdant surroundings (and ritually arrayed animal corpses and other disturbing elements). The set-up naturally recalls The Wicker Man (the British classic more than the Nicolas Cage-starring remake). "Summer" doesn't exactly run away from the comparison, even throwing in a stately manor that's home to the island's patriarch, but it also puts its own spin on its themes of death, rebirth, and the way old, brutal beliefs can linger in ostensibly more civilized times.

Naomie Harris, <em>The Third Day</em>Naomie Harris, The Third Day

"Winter," at first, looks like it's set on a different island and maybe even belongs to a different series, even if the landmarks remain the same. Lowthorpe emphasizes the place's barrenness and characters spend much of their time shivering. Beyond this, the look and feel of Osea — now filled with hateful graffiti and other menacing features — make it seem like a place transformed. But by what? Like much of the The Third Day, that's best left unspoiled, but the decision to split the series this way allows for a fascinating study in contrast that starts with the color scheme an extends to the second half's emphasis on childbirth and motherhood in place of the themes of fatherhood and mating that dominate "Summer." (That it's so abundant in detail makes it easier to fight the urge to yell "Just get off the island!" the longer Helen lingers as the tide starts to recede. If she did the smart thing, it wouldn't be much of a show.)

It's a rich mix of themes and intriguing details intensely played by a remarkable cast (with Considine emerging as something of an MVP by making Mr. Martin so unreadable behind his ingratiating smile). What's not clear, five of six episodes in, is if The Third Day will find a way to bring all the elements together, both narratively and otherwise — and whether all its unanswered questions will end up like dead ends. Will its final revelations will deepen the experience of following Sam and Helen on their troubling journeys — or reveal a series with more on its mind than drawing lines between the bloody practices of the past and the ways we live now? But if it doesn't, The Third Day still makes for a memorably unsettling journey into a dark place, one with an uncomfortable resemblance to the world we already know.

TV Guide rating: 4/5

The Third Day premieres Monday, Sept. 14 on HBO.