S04E11: 'Cabin Fever'

Locke experiences a dreamlike vision of a Dharma Initiative worker called Horace instructing him on how to find Jacob's cabin. When he awakens, Locke stuns Hurley and Linus by heading to a mass grave and taking out a map from the jacket pocket of Horace's corpse.

Meanwhile, the mercenaries return to the freighter and Keamy goes to kill Michael Dawson when he discovers that he was Linus's mole. However, the gun jams and the ship's captain Gault states that Michael must be kept alive to repair the engines he previously damaged. With Gault's help, Sayid takes a boat and heads for the island to warn of another attack. Desmond opts to stay behind.

Frank refuses to fly Keamy and his men back to the island, so Keamy kills both Gault and the ship's doctor until he gets his way. When over the island, Frank discreetly drops a bag into the camp containing a satellite phone.

Locke finds the cabin and finds Jack's supposedly dead father Christian sat alongside his daughter Claire. Christian, who claims to speak for Jacob, warns Locke that an attack is imminent.

In flashbacks, the origins of Locke are revealed. In the 1950s, a 16-year-old called Emily is hit by a car on her way to a date with an older man. John Locke is born prematurely and the baby is closely watched by Richard Alpert, who appears to be the same age as when he appeared on the island in the present day.