S4E5 · The Constant

As the helicopter lands on the Kahana, Desmond slips back to 1996. He is taken to the sick bay, where a crew member is also experiencing "temporal displacement." Sayid calls Daniel, who tells Desmond to find him at Oxford in 1996. He does, and Daniel shows him his time experiments and warns him to find a "constant," someone who is present in both periods of his life, to anchor him to reality. While the ailing man dies, Desmond tracks down Penny Widmore in 1996 and tries to make amends, promising to call her on Christmas Eve of 2004 and leave her alone until then. Back in the present, he makes the call and learns that she has been looking for him since his disappearance. They affirm their love, saving him. Daniel, who suffers from short-term memory loss, finds a note in his journal naming Desmond as his constant.