This episode, unlike most other Lost episodes, does not divide its time between two different time settings, but takes place almost entirely in the past, except for a brief scene at the end of the episode. It depicts the origins of the characters of The Man in Black and his brother, Jacob, and how they came to be on the island. It also reveals the identities the corpses discovered in the cave in “House of the Rising Sun”.

Washed ashore on the island, a pregnant woman named Claudia (Lela Loren) encounters an unnamed woman (Alison Janney). The woman helps Claudia give birth to her son, whom Claudia names Jacob. After the birth of Jacob’s fraternal twin, which was a surprise to both women, the woman murders Claudia. The woman raises the twins as her own, and to be ignorant of a world beyond the island. She also convinces them to be distrustful of humanity, whom she regards as corrupt and dangerous.

At age 13, Jacob and the Man in Black discover that other people are on the island. Mother shows them a mysterious tunnel filled with light and an underground stream. She tells them that one of them will eventually be its caretakers, and that she made it so that they could never kill each other. Soon after, the ghost of Claudia appears to the Boy in Black. She reveals to him that she is his and Jacob’s birth mother and the other people came to the island with her 13 years ago. He confronts Mother, and attempts to convince Jacob to join the other people on the island. However, Jacob refuses and stays with Mother.

Over the course of the next 30 years, Jacob visits his brother at the camp he shares with the other people. While still believing the people are corrupt, he stays with them since they have figured out a way to leave the island by using its strange properties that they found via wells. When Mother learns this, she visits the Man in Black and learns that he and the others have constructed a giant wheel. He tells her that they attempt to dig into the light from the tunnel, existing beneath parts of the island, to create a system that will allow them to exploit its energy, and use it to leave the island. Upon hearing this, she renders The Man in Black unconscious, massacres the people in the camp, and destroys their equipment. She later shows Jacob the tunnel of light again, explaining that it leads to the heart of the island, and that Jacob has to take care of it now. She also warns Jacob never to enter the tunnel, as doing so would entail a fate worse than death. Jacob unwillingly assumes leadership of the island.

Enraged by her actions, the Man in Black kills Mother. Jacob retaliates by beating his brother, and throwing him into the stream leading down the tunnel of light. The Man in Black is sucked into the tunnel and emerges as the smoke monster. Jacob then finds the Man in Black’s physical body and places the corpse with Mother in the cave, along with a pouch containing the black and white stones they used in a game as children.

The corpses and the stones are found centuries later by Jack Shephard (Matthew Fox) and Kate Austen (Evangeline Lily), dubbed by John Locke (Terry O’Quinn) as Adam and Eve. The corpses and the stones will eventually be found by Jack Shephard and Kate Austen years later in the “House of the Rising Sun” episode in season 1.

“Across the Sea” is the fifteenth episode of the American Broadcasting Company’s sixth season of the serial drama television series Lost and 118th and antepenultimate episode overall. The episode aired on May 11, 2010 on ABC in the United States. The episode was written by Carlton Cuse & Damon Lindelof and directed by Tucker Gates.

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