A steamy scene between James and Scott unfolds, but ends abruptly, and Scott tries to leave. Throwing herself between him and the door, James demands he choose: Her, or Chloe. He tries to push past her, but she blocks him, until finally the struggle gets so intense that she brings a lantern crashing down on his head. He drops to the floor, bleeding.
On another part of the ship, Dr. Volker's door refuses to open. Riley happens by and checks the problem, but the door works fine for him. And Greer, patrolling the halls, notices Wray in the gate room. She tells him that she's updating personnel files, and when Greer remains suspicious, she tells him to talk to Young about it - he can answer any questions. But when he does, Wray denies ever having been in the gate room that day.
Volker finds himself trapped again, and this time Scott - in full health - is near. At the sound of Scott's voice, Volker realizes that something was off and goes to lie down, leaving Scott with the image of his own young son.
Perturbed by James's absence, Young sends a team to her room and finds her huddled and crying. They bring her to the infirmary and learn that she thought she murdered Scott. Scott enters, and confesses to his hallucinations of his son, and when an airman is brought in for wounds inflicted while trying to rip a snake out of his own arm, TJ and Young piece together that the most recent away team must have contracted something on the alien planet. They go to find the rest of the away team, including Volker, and bring them to the infirmary. After some detecting, TJ finds a tick-like creature buried in the nape of each patient's neck, but as she attempts to remove it from Scott, he loses consciousness. Chloe is there out of concern for Scott and panics, but TJ and Young manage to revive him. They realize they're going to have to find another way to remove the ticks.
Greer, meanwhile, continues to survey Wray - or at least, what he thinks is Wray - who is acting suspiciously. And Chloe, while heading back to her quarters, discovers her father in an abandoned shuttle, and tearfully embraces him. Before long, Eli overhears her talking to herself, and urges her to see TJ. She tells him she knows that she's bearing a tick, but refuses to go because she wants to spend more time with the vision of her father. She begs him to give her more time, and he hesitatingly agrees. But the medical team soon suspect that the tick jumped from Scott to Chloe, and Scott finds her and carries her to the infirmary.
Soon it's Rush's turn, and he believes his alien abductors are on the ship, pursuing him. As his hallucination intensifies, so does Greer's, and as Rush thinks he's fleeing his tormentors, it's actually Greer, who thinks he's pursuing a mutinous Rush and Wray, and imagines that Young has authorized him to interrogate the two, then use deadly force. Scott goes after him, but Greer has locked himself in the bay with his quarry. Rush, believing that Wray is an alien, stabs her, then knocks Greer down just as Young blows the door open with C4. Rush and Greer are quickly detained, but all are haunted by their fears.