WARNING: The following contains spoilers for The Third Day, now airing on HBO.
The Third Day has been dropping hints since its premiere suggesting Jude Law's Sam has a sinister connection to the island of Osea off Essex. After he loses his son on the English mainland, Sam comes to pay homage to the child at the river he was found, but he ends up saving Epona from hanging herself.
This causes him to head to Osea to take her back to her family, but in addition to being concerned for the teenage girl, a trail of pagan relics, religious rituals, an affair with a woman named Jess and a curly-haired boy who's initially thought to be a ghost have all kept him tethered to the island. However, in "Sunday -- The Ghost," on his third day here, viewers discover Sam does belong in Osea after all; although, he's forced into fulfilling his destiny thanks to a nasty betrayal.
Sam's tries to escape the island after being attacked, but the Martins reveal his grandfather wasn't stationed on Osea during the World War II; he was actually next in line to be the "Father," aka the island's leader. He's a descendant of the Charrington bloodline, but his grandpa fled as he didn't like the Celtic blood sacrifices. Since then, the people have been trying to get this bloodline back.
Sam's grand-uncle, the mysterious white-haired old man he's been hallucinating, is actually real and the current leader; although, he's brought the island nothing but blight. It's why the Martins orchestrated to kidnap Sam's son, Nathan, so they could groom him for the future. They sent a decoy body to make it look like Nathan got killed, so when Sam assumes he's seeing a ghost out of grief, it's really Nathan. It seems Osea's Celtic spirits have drawn Sam home as Nathan isn't ready to be the leader, and someone needs to replace his grand-uncle.
However, Sam doesn't believe this and makes a couple attempts to escape. The first sees him almost drowning on the causeway. The second occurs when he flees Mrs. Martin, who believes that killing a reluctant leader will be the ultimate sacrifice that makes the land and sea bountiful again.
Sam eventually gets on a boat with Jess, but she shockingly pulls a flare gun on him and turns the boat towards the shore. Sam's stunned as he thought she wanted to get back to her daughters, especially after she grew up in a cult too. However, when they get to land, the two blonde girls Sam's been seeing with the old man rush to greet Jess.
They're her daughters, held hostage by the Osea clan and used as pawns, indicating her estranged, abusive husband set this role out for them. Sam is worried because the Martins, as well as all other inhabitants, are there to greet him, making it seem like it's his funeral. His grand-uncle approaches with a gun, but after thanking Sam, he kills himself, officially passing the mantle of "Father" to Sam.
The islanders clap as he's the rightful heir and this is how it should have been all along. As Sam walks to the Charrington mansion while having a tearful mental breakdown, Nathan runs out to meet him. They hug and go inside, and crickets swarm the screen, which seems to indicate Osea is happy it got its king back.
Starring Jude Law, Naomie Harris, Katherine Waterston, Nico Parker, Charlotte Gairdner-Mihell, Paddy Considine and Emily Watson, The Third Day airs at 9 p.m. ET/PT on HBO.