Vidiian organ harvesters steal Neelix's lungs, and the Doctor must keep him alive while the crew hunts down the thieves.
Neelix is attacked and loses his lungs to a Vidiian, one of a species suffering from a disease known as the phage that forces them to harvest organs from other peoples. The Doctor keeps him alive, and Kes volunteers a lung of her own as a transplant, a decision that has consequences for the bond between the two.
The episode introduces the Vidiians, who would return as recurring antagonists in the first two seasons, and it gives the show its first real horror-tinged threat. It is also a significant test for the Doctor, a hologram who is forced into a medical emergency far beyond his usual duties.