Voyager discovers an alien creature which resembles a large colorful scorpion, at least a metre in length. It is wounded and the crew brings it to sickbay to care for it. B'Elanna Torres examines the alien, which suddenly leaps and attacks her. It punctures her body and attaches itself securely, forming a physical and biochemical connection with her.
The Doctor finds that the chemical attachment between B'Elanna and the creature is too complex to break. If he were to separate them he surmises that B'Elanna might die. He searches the computer database for any information that might help him. He finds a file on a brilliant Cardassian exobiologist named Crell Moset, and activates a hologram of the scientist. He and Moset experiment with procedures on the holodeck. They vivisect a holographic recreation of the alien creature, discovering that its neuroanatomy suggests intelligence. The Doctor understands he must remove the creature from B'Elanna without killing either of them.
As the crew hear about the situation, a young Bajoran ensign named Tabor protests angrily at the Doctor's use of the Crell Moset program. He explains that during the Cardassian occupation of Bajor Crell Moset used living Bajoran prisoners as test subjects, and that he killed and mutilated many. B'Elanna is dying and barely conscious but she refuses to undergo any procedure that has anything to do with Crell Moset. She would rather die than benefit from knowledge obtained from killing. Captain Janeway orders the Doctor and the Cardassian hologram to operate on B'Elanna against her wishes.
The two holograms administer a non-lethal shock to an area of the creature's nervous system that causes it to withdraw its connection to B'Elanna. During the procedure the alien's companions arrive in a ship and hold Voyager in a tractor beam. Janeway sends the creature to the ship and it releases Voyager and leaves the area.
Captain Janeway leaves the decision as to whether to keep the holographic program of Dr. Crell Moset in the computer databanks, in case of future emergencies to The Doctor. Janeway's next challenge will be to confront B'Elanna, who is utterly furious at having been denied the right to refuse treatment.
The Doctor, ethically opposed to the methods used by the real Moset to gain his knowledge, deletes the holographic program and "all related files".