Warning: Spoilers are ahead for The Third Day finale.
It is easy to get sucked into the chaos of The Third Day. To want to solve the mystery behind Osea's religious cult or figure out whether the island is really the "soul of the world." But in The Third Day finale, it becomes clear that for all Osea's sacrifices, both figurative and literal, the HBO miniseries works best as an allegory for grief.
The final shot of "Last Day — The Dark" is of the sun coming up on Helen (Naomie Harris) and her daughters, who have narrowly escaped the island with their lives. In fact, we don't actually know if Helen, who swam the length of the causeway in freezing cold water, will survive. In some ways, that doesn't matter. Together the three of them have managed to stave off the darkness for another day. They have taken one giant step out of the pit of despair which has swallowed Sam (Jude Law) whole.
Throughout The Third Day, we see Osea through Sam's eyes and understand the death of his son Nathan through his experience. This is not to say we take him at his word, but his pain makes him a sympathetic figure. But as he says in "Summer," "grief is bespoke" and Helen's is far different than his.
As is her understanding of what happened the day Nathan disappeared. She reveals that Nathan wandered off while Sam was on the phone. Not for a few seconds, as he claimed, but a 23 minute and 17 second call to "one of his flings" who he was trying to break up with. "You lost my son," she tells him.
Helen did not just lose her son that day, she also lost the man she thought was her partner. She didn't come to Osea looking to bring Sam home, but to get the money he had stolen so she could start a new life with the girls. She is looking to move on — she reveals he won't sign the divorce papers — but he is stuck in the past, in the pain. It's why he believes this little curly haired boy on the island is Nathan, despite the fact that the real Nathan would be 16 by now. This is "another boy altogether," Helen yells at him. "Jesus Christ, he's not the right color."
This boy, as Mrs. Martin reveals, is Sam's grand uncle's child. He is Sam's blood, which might be why he feels so familiar, but, honestly, all they needed to find was a boy with curly hair. It's the feature Sam points to in hopes of convincing Helen he is really their son. He can't come to terms with what happened and his role in it so he's spent the last decade coming up with explanations that will absolve him of his guilt. He blamed others for what happened to Nathan so he would never have to take responsibility for anything he did. His latest explanation is that a stuck in time Nathan has been trapped on Osea.
To Helen, it sounds outlandish and infuriating that Sam could believe this boy who looks nothing like their son is Nathan. But those who have dealt with grief know it is all-consuming. Sam's desire to have him back has made it impossible for him to accept that he is gone. He has been overtaken by his "insane operatic grief," as Helen calls it, and become someone else entirely. Osea allows him to become the person he wishes he was: a protector, a Father, a savior. But he can't stave off the darkness that so many on Osea warned him of. In the finale, this violently comes to light.
The Third Day was never really about an island, it was always about grief and how it manifests in us. Sometimes it makes you feel as if you are trapped on an island of your own pain. Sam finds solace in Osea being a place where he can continue to grieve alongside others who have lost. But Helen understands that she has to keep going, keep living, keep moving forward. It's why she has buried her grief deep inside, believing it is the only way she will survive.
There is no one way to grieve. People process their pain in different ways. What works for Helen, does not work for the others in her family. Ellie (Nico Parker) finds it unsettling how Helen will not even say Nathan's name, as if she would prefer to forget him all together. Helen believes that this is her being strong for her girls, despite the pain it is causing them. She must not show her own pain or the guilt she has over that final conversation she had with Nathan. She told him she wished he hadn't been born. She didn't mean it, but she never got to tell him that. Now she has to live with that forever. It's why she must keep Nathan's memory locked away so she doesn't get bogged down by the sadness she feels every time she thinks of him. She's seen what this has done to Sam, who can't escape Osea even if he tried.
Helen describes herself as a non-believer, who finds Ellie's interest in organized religion concerning. But what she learns from being on Osea is that we all need something to believe in. It doesn't have to be religion, it can be something different, something that, like grief, is also bespoke.
When she's pulling her daughters back to the mainland she sees Nathan. There in the sea where he was buried a decade ago, calling out to her. It is really him, not the little boy from the island who almost made her believe he could be her little boy after forgiving her for what she said all those years ago. Could fake Nathan be the future Father of Osea? The Christ-like figure who could restore Osea's soul.
Helen will never know. While Sam chose to follow Nathan, she forges ahead, away from Osea, without him. It isn't easy, but she knows that to stay with her son means certain death for her and her girls. She must stay alive for them. Making that decision is the first step towards healing. Grief is not a one and done thing, it's a day to day process full of highs and lows. The fact that Helen chose to keep going is a sign that she believes there is something more out there. In the end, that is enough.