The crew finds themselves part of an experiment.
Crew members suffer strange ailments, from Janeway's severe headaches to Chakotay rapidly ageing, and Tom Paris and B'Elanna Torres's new romance is unusually intense. The Doctor and Torres find alien markings on the victims' DNA, but before they can learn more, Torres collapses and the Doctor is shut down. He secretly adjusts Seven of Nine's ocular implants, and she sees cloaked aliens moving among the crew, studying them with devices like lab animals.
Janeway, pushed to the edge by the experiments, steers Voyager between two pulsars that will crush the ship unless the aliens leave. They flee to their own vessels and the crew survive by a narrow margin, and the Doctor reverses the changes. The fourth-season episode is a story about the ethics of scientific study, told from the point of view of the test subjects, and it ends with Paris and Torres deciding to stay together anyway.