WARNING: The following contains spoilers for The Third Day Episode 5, "Tuesday -- The Daughter," now airing on HBO.
HBO's The Third Day has placed Helen's family in huge danger now that people know she's actually Sam's wife Cas, who's come to get him back from Osea Island. Unfortunately, Jess has given birth to Sam's newest daughter after a fling, and she's ready to kill them since she wants her daughter to be next in line for the throne.
Additionally, the island is going through a civil war as some endorse Sam's bloodline ruling, while others don't. This means Jess will have followers who'd prefer a Charrington born on the island as opposed to outsiders. However, Helen may find her family being slaughtered by a surprising enemy, as her own daughter, Ellie, is being corrupted.
It's not far-fetched to see Ellie cracking as Sam has a history of mental breakdowns, stemming from his ancestor, Frederick, who was the founder of Osea and Jack the Ripper. Fans think Frederick's bloodlust has crept down the family tree, and Ellie's the latest one susceptible due to her beliefs. Frederick believed he was appeasing the gods and balancing the world through human sacrifices. While it's unclear how far gone Sam has become in his role as "Father" of the island, Ellie may be more open-minded to murder as she learns the island's dark history with sacrifices.
Ellie has taken to Christianity a lot, which Helen doesn't like, berating Ellie for this. Her own sister, Lu, also makes fun of her, which is why Jess' older daughter, Kail, latches onto Ellie. Kail butters Ellie up, agreeing with her that they're children of Heaven who will be saved when judgement day arrives. Some think Kail is using Ellie to lure her in as a sacrifice, but this could also be a form of conversion, luring Ellie into serving Osea's Celtic ways, which are a warped form of paganism and Christianity.
Kail could want Ellie to be one of them, but there's probably more to it. She knows Ellie's bullied back home and is an outsider. She also knows Ellie wants to do the right thing, much like Frederick back in his youth, making Ellie the perfect person for something sinister on behalf of the people of Osea.
The darkest clue to support this comes when Kail calls Lu a blind sheep, and as it was established in the prior episode, lambs are often slaughtered for rituals here; however it's believed this is necessary for the island to prosper. As Kail tells Ellie, once this happens and Osea heals, so will the world.
If not, disease and war breaks out, with Kail listing historical events, appealing to Ellie's compassion, empathy and kindness. She's clearly manipulating her into joining the cult so catastrophes don't occur. This foreshadows Ellie possibly doing the unthinkable and killing Lu or her mom, as she views them as heathens who could threaten the balance. If their deaths can help make the world right, then she has reason to kill them. It doesn't help that her family is so disconnected from Ellie, which is made even worse by Kail being a kindred spirit of sorts, one who wants Ellie to embrace the bloody legacy of the Charringtons, which Sam may be doing too.
Starring Jude Law, Naomie Harris, Katherine Waterston, Nico Parker, Charlotte Gairdner-Mihell, Paddy Considine and Emily Watson, The Third Day airs Mondays at 9 p.m. ET/PT on HBO.