Clarke, Bellamy, and a group of Delinquents investigate the Exodus dropship crash. There are no survivors.

Raven tells Finn to help her find the black box to explain why the ship crashed.

Bellamy warns everyone to keep on the lookout for Grounders in retaliation for the Unity Day bridge battle.

Clarke discovers a pink, smelly liquid that Raven tells her to get away from because it is highly explosive hydrazine. She demonstrates by throwing some and creating a small explosion and tells Bellamy to clear the area.

Meanwhile...

Back at camp, Octavia joins Monty at a lookout spot on the fence and she spots a white flower outside camp walls, Lincoln's signal for her. Jasper is behind them, regaling some of the other Delinquents with his story about how he killed two Grounders at the bridge meeting. Octavia is irritated by it and Monty tells her to let them enjoy it.

Something hits the tripwire surrounding the camp and Octavia worries it might have been Lincoln. Derek sees something move and starts shooting at it.

He and Connor then go to check what it is with Octavia getting there first because she thinks it's Lincoln. It turns out that a badly beaten and tortured John Murphy has returned to their camp after having been banished by Clarke and Bellamy, due to trying to get revenge on a girl named Charlotte. John days earlier was framed for a murder he didn't do, Charlotte was the one who killed Wells, aka The Chancellor Jaha's son.

Murphy arrives back at the camp with a story of captivity and torture at the hands of the Grounders. This sparks conflict between Clarke and Bellamy, who have differing views on how to deal with his return and the potential implications for their relationship with the Grounders.

Murphy is brought into the drop ship and Bellamy comes looking for him when he returns to camp. He orders everyone but Connor and Derek out of the drop ship.

Murphy tells them he was running from the Grounders. Bellamy starts to point a gun at him for returning and Finn intervenes, telling Bellamy that Murphy might be able to tell them things about the Grounders.

Bellamy tells Finn to get out of his way but Clarke tells him Finn is right. She sees that Murphy's fingernails were torn off and that he was tortured.

Bellamy wants to know what Murphy told the Grounders and Murphy admits, "everything." Clarke tells Bellamy to let Murphy get better and then forces him to leave again and if he doesn't, then they kill him.

Just a bit later, In the drop ship, Murphy is puking up blood.

Clarke asks him how he escaped and he tells her they forgot to lock his cage so he ran off.

Clarke realizes the Grounders purposefully let him go just as Bellamy enters the drop ship, asking what is happening.

Clarke tells him it's biological warfare. Bellamy blames Murphy but Murphy tells them he didn't know. Derek starts seizing before collapsing and dying. Clarke orders a quarantine. And so the virus spreads...