Seven of Nine watches as Rebi and Azan, two of the Borg children she had been taking care of since they were separated from the Collective, are reunited with their people, who agree to also give Mezoti a home. The only one of the Borg children staying aboard the U.S.S. Voyager with Seven is Icheb, the eldest of the four. After the children leave, Icheb notices that Seven is crying. Seven claims that her ocular implant is simply malfunctioning.
When Seven goes to see the Doctor, he confirms that there's a glitch in her cortical node, and he gets her to admit that she has been having headaches. Later, when Seven attempts to regenerate, she discovers she cannot interface with her alcove because her cortical node is malfunctioning. She stays up all night in the mess hall until Neelix arrives and offers to make her breakfast. Seven then falls to the floor convulsing, and Borg implants start bursting through her skin.
In sickbay, the Doctor realizes Seven's problem is more serious than he previously thought. Her cortical node, which regulates her vital functions, is destablizing, which means she will die unless it can be replaced. Captain Janeway gets an idea: She orders Ensign Kim to scan a nearby Borg debris field and transfer the coordinates to the Delta Flyer — she plans to salvage a replacement cortical node. Janeway, Lt. Paris and Tuvok depart for the debris field where they find several dead Borg drones and manage to remove the cortical node from one of them. Back on Voyager, Janeway and the Doctor practice the cortical node replacement operation several times in a holodeck simulation, failing miserably each time. They eventually decide that they cannot use a cortical node from a dead drone, but only from a living one.
Icheb also comes to realize that only the cortical node from a living Borg can save Seven, so he volunteers to have his own node removed and transplanted to her. Icheb devises a plan to have himself genetically altered to survive without his node, but the procedure is risky. When Icheb cannot get Janeway, Seven or the Doctor to listen to him, he programs the computer in his regeneration alcove to disengage his cortical node. Now dying, Icheb eventually convinces them to give his node to Seven and perform the genetic procedure on him. The operation is successful and both Seven and Icheb fully recover. Seven offers to help him study for the Starfleet Academy entrance exam, promising him a rigorous schedule. But then Icheb notices another tear in her eye ... only this time it isn't a malfunction.