Abby is determined to find a way to protect her people from a coming attack by the Grounders, even if it means heading into unknown territory and leaving some captives behind. A worried Bellamy convinces Clarke to go with him to the drop ship, where he and Octavia reveal a dangerous secret. Clarke comes up with a bold new plan to stop the Grounders' attack, leading Abby to a desperate decision. Meanwhile, at Mount Weather, Jasper and Monty begin to question the motives of their new friends, while Dr. Tsing conducts a deadly experiment to determine how to make use of the captives.