Contains spoilers for this episode and earlier episodes.
A pregnant shipwreck survivor, Claudia, gives birth to twins on the island. A woman who helps with the delivery kills her and raises Jacob and his unnamed brother as her own sons. She keeps them away from other people and shows them a cave filled with light, warning that the light must be protected. Jacob's brother sees Claudia's ghost and learns that the woman they call Mother lied about their origins. He joins the other castaways, determined to leave the island, while Jacob stays. Years later, the brothers still meet, but disagree over whether the people living there are as corrupt as Mother claims.
Jacob's brother works with those people to reach the island's energy through wells. He shows Mother a wheel intended to channel water and light so that he can escape. Mother knocks him unconscious; he wakes to find the settlement burned, its inhabitants dead and the well filled in. Mother meanwhile makes Jacob drink from a cup and accept responsibility for guarding the light. His brother kills her in retaliation. Enraged, Jacob drags him to the source and sends him into the stream, after which black smoke emerges. Jacob finds his brother's body and lays it beside Mother's in their cave, with the black and white game stones. These are the remains the survivors later call Adam and Eve.