WARNING: Spoilers for The Third Day ahead
In HBO's limited series The Third Day episode 4, "Monday - The Mother", Helen (Naomie Harris) comes to Osea with her daughters, looking for her husband, Sam (Jude Law), but it's possible she might also be connected to the island and unable to leave.
Helen travels to Osea at first under false pretenses: she's trying to celebrate her daughter's birthday by indulging her interest in archaeology. However, the disgruntled preteen insists that she's no longer interested in archaeology, and her repeated insistence serves to further state that Helen's motivations likely run much deeper than what's presented on the surface. The ending of "Monday - The Mother" shows a family photo with Helen and her family, which includes the primary protagonist of the first three episodes, Sam. Sam initially came to Osea to grieve his murdered son, Nathan, who he sees in what appear to be strange visions as he spends more time on the island. Sam mentions his wife and two daughters repeatedly, but has an affair with Jess (Katherine Waterston).
The first three episodes of The Third Day largely tell Sam's story, and reveal that he's destined to become the Father of Osea. He's linked to the founder of the island, and was always meant to come home. Osea seems to have a greater influence on the rest of the world, which is why it's strange inhabitants are so keen on keeping the status quo and their strange traditions. It's said that Osea is the "soul of the world"; if the island is sick, the whole world will be sick. While it could just be a strange belief system or urban legend, the islanders take their role very seriously and, while Sam is hesitant to take on his birthright as Father at first, he later accepts his place in "The Big House" when he's told that he can be reunited with his son. Similarly, the final three episodes seem geared toward Helen's story, which could mean that she's meant to be Osea's "Mother" and reunite with her family — for better or worse.
Helen & Sam Came To Osea Island For Similar Reasons
Sam and Helen both seemed to be drawn to Osea to find something precious to them that was lost. In Sam's case, he was going to grieve Nathan's death and find answers—which he seems to hope will lead to closure—about the event. Helen is seemingly looking for answers about Sam's disappearance; his behavior was increasingly erratic before he left with a large sum of money, and the fact that he'd abandon his remaining family seemed suspect. Helen shows determination and grit from the beginning, outwardly refusing to be turned away when she tries to get the island's residents to honor her Air BnB reservation for her family vacation, and even going so far as to break into someone's house for shelter. She's relentless in her pursuit of answers, a far cry from Sam who wanted solutions, but was too caught up in his emotions and his affair with Jess to go after what he desired with the same tenacity.
Helen, as a mother, seems bound to keep her family together at any cost, but is taken aback when she helps Jess give birth to her daughter, only to discover that her new baby is Sam's child. Though obviously betrayed, Helen goes to "The Big House" to find and confront Sam, and after the events of The Third Day episode 5, "Tuesday - The Daughter", she and her husband are reunited. Sam doesn't seem surprised to see her; in fact, his reaction is rather calm and even quietly accepting, even though he's got to know she's rightfully upset with him for running away, leaving her to care for their daughters. On top of that, he's disrespected his marriage by committing adultery, and Helen is rightfully upset. Even so, Sam's reaction could be one of welcome — especially if Helen and their daughters are also meant for Osea, as he was.
Helen Is "The Mother" & Sam Is "The Father"
Just like The Third Day's episode titles suggest, Helen could be "The Mother" of Osea, and Sam is already "The Father", as his episodes suggested. Much of The Third Day is left in detailed symbolism, but applying logic would draw a conclusion that Sam's children—since he's biologically related to Charrington—would also inherit a stake in Osea's strange oligarchy. Helen is his wife, so she's not biologically destined or connected, but if Sam wants to keep their family together, it would make sense for her to assume "The Mother" position, if one exists. As in nature, there's a lot of masculine and feminine balance, and Osea seems to assume more Pagan traditions and symbolism, despite some of the islanders previously claiming to practice a Christian faith.
Despite the family thinking Nathan is dead, it's quite possible he isn't — or, if he was actually killed, his ghost or spirit remains connected to the island somehow. The various traditions of Osea Island reflect other folk horror sensibilities, such as Ari Aster's Midsommar, where death is accepted and even graceful. It's encouraged for necessary reasons, and while shocking, senicide (the killing of the elderly) is committed without a second thought, if it's meant to serve a greater purpose. Just as the seasons must shift, life and death are also cyclical, so it's possible that anyone who is directly related to Charrington will eventually return home and reside on Osea in death, even if they weren't directly part of the island during their life.
Helen's choice to bring her family to the island could have also been fated; destiny might be trying to reunite Sam's family and give them a chance to forgive and start over. If Osea Island has to be kept healthy and thriving in order for the rest of the world to function the same way—which seems to be what the majority of the islanders believe—then it makes sense for fate, destiny, or even the islanders, who have already proven they'll intervene at any personal cost to achieve their goals, to want to bring them back together. Jess' giving birth to Sam's daughter certainly adds an extra layer of strain on his marriage to Helen, who likely won't easily forgive, but the traditions of Osea could be one where the Father could have multiple wives in order to continue his lineage and ensure Osea thrives for generations to come. The Third Day has yet to reveal the last of Osea's mysteries, but it seems likely that Helen's journey across the causeway wasn't just coincidental or for a archaeology trip, and was likely more than the simplicity of wanting to find answers suggests.