A young girl is found at the outskirts of Terra Nova and she is brought into the colony. At the clinic, she panics and tries to escape but Elisabeth manages to calm her down. The girl's name is Leah Markos, and she claims to have run away from the Sixers. She wants to return to 2149 and is upset when she's told that this is not possible.
Taylor wants to keep Leah around (even though she doesn't trust him) in order to find out more things about the Sixers and their camp. Elisabeth says that Leah can stay with the Shannons. Leah dropped her bag out in the jungle and wants it back. Washington takes one other soldier and goes in search for it. They find it, along with a Sixers trap.
Maddy is eager to start her apprenticeship with her mom at the clinic, and Josh is charged with babysitting Zoe and Leah. Maddy changes her mind fairly quickly, however, when she realizes she doesn't have the stomach to deal with gory injuries.
A group of Sixers arrive at the colony gate. They have Washington and the other soldier and wish to trade them for Leah. Taylor demands Mira release the hostages but they should allow Leah to decide whether she wishes to leave. Leah wants to stay. Taylor is angry at the number of times Sixers have come to Terra Nova. Once more, he says, and it will be war.
To gain more information about the Sixers, Jim talks to Taylor about the Sixth Pilgrimage. Taylor says only around half of the colonists left with Mira and the ones left were scrutinized. Maybe they glossed over something then which might be important now. They accuse a scientist of going outside of Terra Nova alone, though he denies being in contact with the Sixers.
Leah is sent to school, but she runs away anyway. Young soldier Reynolds talks to Jim about getting permission to court Maddy. Jim is a little shocked at the request but he has other things to worry about when a home break-in is reported. Someone had dug through the floor of the house, unearthing something buried there. Jim finds out that the house used to belong to Mira.
Leah is found carrying a strange container in her backpack. No one is able to open the object, and Leah claims not to know either. Mira has her brother, her only family, and she had to do what Mira said so that she'd get Sam back unharmed. Jim wants to go rescue Sam, but Taylor refuses to let him leave the colony. What if Leah is lying about her brother, too?
After Maddy'sunceremonious departure from the hospital, Reynolds asks her for permission to start courting. He says that Taylor insists that they should be this formal. Maddy agrees.
Jim heads out on his own and is ensnared in a Sixer trap. While he's suspended upside-down above the jungle canopy, a dinosaur nearly makes a meal out of him, but Sixers scare the creature off. Jim is knocked unconscious and taken to the Sixers' hidden camp.
Mira claims that she only told Leah she'd hurt her brother so that the girl would be more cooperative. Sam is safe. Jim tells Mira that he believes the Sixers have a spy in Terra Nova. Mira says that it was lucky Jim saved Taylor's life because the last thing she wants is to make Taylor a martyr. Some very powerful people back in 2149 wish for Taylor to be removed from power. Mira is willing to help them because they have her daughter.
When Jim asks "Who's 'they'?", Mira refuses to answer and allows Jim to leave, along with Sam. Leah and Sam are reunited at Terra Nova, and a colonist has agreed to take them into her home. Elisabeth tells Jim that Maddy isn't cut out for medical duty. Malcolm continues to examine the strange container, but is still unable to open it. Jim says that he should keep trying.