A peaceful day of exploration turns deadly when a sudden explosion traps Marguerite and Roxton in an underground chamber. Their only hope for escape is Challenger, but the same explosion has left him with no memory of who he is, or what has happened to his friends. Meanwhile, a contest of friendly rivalry between Veronica and Finn leads to an enigmatic discovery at the Treehouse, as another memory from Veronica's childhood is literally uncovered. Faced with certain death, Roxton and Marguerite reveal emotions best left buried, so that even if they do survive their ordeal, there's a chance their relationship will not.