Kes lives her life in reverse.
Kes wakes as an old woman in sickbay, surrounded by children and grandchildren she does not know, and then finds herself jumping backwards through her own life. The Doctor discovers she has been exposed to chronitons, and Kes slowly pieces together a future in which Voyager survived a Year of Hell against the Krenim, a war that killed Janeway and Torres and left the ship badly damaged.
As she moves further back, Kes works with the crew of each period to understand the Krenim chroniton weapons and the cure, a biotemporal chamber. She regresses all the way to infancy and before birth before the treatment takes hold and she returns to the present, about three years after she joined the ship. The third-season episode looks ahead to the fourth-season two-parter Year of Hell, though events there unfold differently, and it ends with Kes deciding to write down what she learned about the Krenim.