Scott and James have sex in her quarters. Once they're done, he dismissively prepares to leave her and go back to Chloe. James insists that he choose between her and Chloe. He ignores her and goes to the door, and James clubs him over the head from behind, killing her.
Volker prepares to leave his quarters but the door won't open for him. He bangs on the door for help and initially gets no response. Finally, Sgt. Riley arrives and opens the door. When Volker complains that the ship's controls are malfunctioning, Riley tests the door and shows him that it works fine.
Greer is walking by the Gate Room when he notices Camille working at the computer. He asks her why she's alone and she claims that no one was there when she arrived. When Greer wonders what she's doing, Camille says that she's working on the personnel files and that Rush and Young know what she's doing. She leaves and Eli arrives, asking Greer if he's seen Scott.
Young calls James on the radio and calls her to her assignment. She apologizes for being distracted, and stares at Scott's corpse.
In the mess room, the soldiers are talking about how Young called off the search for the missing Franklin. Airman Dunning studies his arm and scratches at the skin nervously.
As Volker walks to the lab, he suddenly finds himself sealed in by piping no matter where he turns. Scott, alive and well, walks by and the piping disappears. Scott sees nothing and suggests that Volker lie down. As Scott continues on, he sees his son Matt standing in the hallway ahead.
Rush goes to see Camille. Young finds them and asks Camille about Greer's claim that she was in the Gate Room alone. Camille denies it and blames Greer for being paranoid, but Young notes that the solider has good reason after Camille and Rush led the mutiny.
Scott chases after Matt and runs into Chloe. She didn't see a boy run past her and doesn't know what Scott is talking about. Scott is called to the workout room where Dunning has gone berserk. The airman is trying to cut open his arm and is yelling that there's a snake beneath the skin. Scott helps subdue him and notices Matt standing in the doorway.
Johansen checks Dunning but can't find any reason for his hallucination. Young tells her to keep Dunning under observation.
In the mess room, Eli and the others discuss what happened. Eli is relieved that Dunning didn't attack anyone but himself. Riley figures that they're lucky that no one else is cracking up from the continual stress. Chloe watches as Greer stares nervously at Camille and Rush, working together at another table. When she asks if he's okay, Greer says that he's fine.
Later, Rush is working alone and hears Greer behind him. He goes out to investigate but finds nothing. Brody notices and asks who he's looking for, but Rush says that he was mistaken.
Volker is trying to sleep when he suddenly finds himself entombed in a coffin. He bangs and tries to get out without success.
Scott keeps seeing Matt, who keeps walking away from him.
When James fails to show up for her assignment, Young goes to her quarters to see what's wrong. He finds her sitting on her bed, crying. James begs for his forgiveness for killing Scott, but Young has no idea what she's talking about.
Scott finally catches up to his son and touches him. He seems real, but Scott insists that he can't be there. Matt asks why his father left him. Before Scott can respond, he's called to the infirmary. James is there, and shocked to see that he's still alive. Young and Johansen inform him that James and Dunning are suffering from hallucinations, and Scott admits that he's been seeing his son. They realize that Scott, James, and Dunning were all on the last away team, and that Volker and Barnes were also on the mission. They go to Volker's quarters and hear him screaming in terror.
Johansen checks on Barnes, who says that she's fine. Young puts the away team into quarantine. As he walks down the hallway, Chloe approaches him and asks what she can do to help. Young assures her that they've done everything they can. Johansen calls Young back to the infirmary and Chloe goes with him. The medic informs them that she's discovered a tick attached to the neck of each away team member except for Barnes. Scott volunteers to have Johansen try and remove it normally. When she does, his heart stops and she and Young manage to revive him. However, the tick has dug in even deeper. While Johansen tries to come up with another way to remove the tick, James hallucinates Scott covered in blood.
Greer continues to watch Camille, who is covertly doing something with the ship's systems.
Chloe walks by the shuttle and notices someone seated inside: her father, Senator Alan Armstrong. He greets her and says that he's missed her.
Greer finally confronts Rush and Camille as they work together on a secret project. They arrogantly deny doing anything, but Greer insists that they're not going to start another mutiny. Rush sarcastically dismisses him as paranoid and they walk away.
Chloe tells the senator what has been happening since he died and that she's used the communication stones to talk to his wife and tell her that he's dead. The Senator says that he's happy that she's managed to become an explorer like she always wanted.
Johansen tells Young that she has a possible cure: the plant-based anesthetic she developed to sedate Rush when she had to remove the tracking device from his heart. However, when she checks Scott again, she discovers that the tick has left him. The ticks on the other victims are still there, and Johansen speculates that Scott's tick jumped to someone else.
Eli goes to the observation deck and sees Chloe talking to her imaginary father. She tries to explain and admits that she knows that he's not real. Eli tells her about the tick infestation, but Eli says that she doesn't want to be cured since she knows that she's hallucinating and it's not harming her. She makes Eli promise not to tell anyone so that she can have the time with her father.
As Rush walks the corridors, he keeps seeing Greer behind him. He dares Greer to step out, but gets no response. Rush walks away but finds himself suddenly trapped behind glass panels. Water floods the resulting chamber and Rush starts to drown... and then realizes that he's hallucinating as things revert to normal for him.
Johansen tries to find the missing tick but no one is infected. However, she remembers that Chloe was in the sickbay earlier and Scott goes to find her.
Rush takes refuge in his quarters but sees an alien come inside. He manages to trap it and flee outside.
Johansen operates on James, applying the anesthetic to the tick and safely removing it.
Rush tries to call Young but gets no response, and is forced to go back into the corridors.
Scott looks for Chloe and finds Eli, who shows him to the observation deck where Chloe is sitting. When Eli breaks his promise and Scott realizes what's going on, he insists on taking her to the infirmary. Chloe insists that she's okay and her father tells her not to go, but Scott and Eli take her away.
Rush goes to the Gate Room and sees the aliens arrive through the wormhole. They spot Rush and come after him.
Greer confronts Camille, who warns him not to lie about her to Young. He starts to argue, but then sees Rush fleeing and goes after him.
In the infirmary, Chloe insists that they let her keep the tick, but the others warn that they have no idea what it could do to her. As Johansen sedates and remove the tick, "Senator Armstrong" tells his daughter that he loves her, and that he's proud of her for living her dream. When the tick is removed, he disappears. Greer calls in on the radio and tells Young that Camille and Rush are preparing another mutiny. Young tries to get through to him but Greer hallucinates an entirely different conversation where Young tells him to shoot any mutineers and maintain radio silence. He cuts off and Scott wonders how his fellow soldier could be infected. Johansen concludes that some of the victims brought back more than one tick, and begins a check of the entire crew.
Camille goes after Greer, who is following his hallucination of Rush. Meanwhile, the real Rush takes refuge in a storage room. He grabs a tool and attacks one of the aliens, only to discover that it's Camille, who followed Greer into the storage room. Rush stabs her in the shoulder and then Greer knocks him down. The soldier seals the storage room door and then points his gun at Camille.
Scott and his team track Greer, Rush, and Camille to the storage room. While Young goes to help after making sure Johansen is okay, Meanwhile, Volker speculates that the ticks dose the limbic part of the brain with venom, and those with stronger emotions suffer more potent hallucinations. James looks at Chloe and considers her own feelings for Scott.
Camille begs Greer for medical attention, and then points out that he's talking to a radio that has been shut off. He insists that he's talking to Young and warns his superior that Camille won't talk, and he should eliminate her and Rush.
Scott and his team get to the cargo bay. When she hears them, she yells for help. Greer ignores Scott, who prepares C-4 to get through the locked door. As Greer prepares to shoot Camille, a hallucination of his father Reginald appears and encourages his son to do it. As Greer prepares to pull the trigger, Rush attacks him. As the two men struggle, Scott blasts the door open. The shock separates them and Scott's soldiers subdue Greer. Rush goes berserk, thinking they're aliens, and they're forced to subdue him as well.
Later, Johansen cures the remaining victims and gives everyone a clean bill of health. When she says that things should get back to normal, Camille wonders what that means any more.
As the crew tries to get on with their lines, James watches Scott. Rush still looks around nervously, watching for the aliens. Camille pointedly ignores a sheepish Greer. Chloe goes to the observation deck, alone, and Eli watches her silently.
arrives at the next planet, Young prepares a team. He wonders if it's safe and figures their luck has to change sometimes, and tells Scott to take his people through.