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 Cardassianexobiologist 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, the real Moset used living Bajoran prisoners as test subjects, and the 'treatment' killed and mutilated many. B'Elanna is dying and barely conscious, but she refuses to undergo any procedure that has anything to do with Moset. She would rather die than benefit from knowledge obtained from the Cardassians, especially one who tortured Bajorans. Captain Janeway orders the Doctor and the Cardassian hologram to operate on B'Elanna anyway, against her wishes.
The two holograms operate on the creature, attempting to remove it from B'Elanna. Crell Moset advocates using a frequency sure to free B'Elanna of the creature, at the expense of the creature's life, but the Doctor objects. He instead decides to 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, which releases Voyager and leaves the area.
Captain Janeway allows the Doctor to make the decision as to whether to keep the holographic program of Dr. Crell Moset in the computer databanks, in case of future emergencies. Janeway's next challenge will be to confront B'Elanna, who is utterly furious at having been denied the right to refuse treatment.
Following the highly successful surgery, the Doctor confronts Moset on the holodeck. Moset does his best to convince the doctor that Machiavellian methods are the only thing that can save the Voyager crew when stranded so far away from Earth. Moset even asks how many lifeforms on Earth were saved through the experimentation of lower beings. The Doctor indignantly counters with "BUT NOT ON PEOPLE!" Moset furthers his plea by quoting the Hippocratic Oath to The Doctor with the line, "Do no harm". The Doctor says it back with disgust, and finally decides his only course of action is to delete Moset and all related files. The Doctor watches as both Moset and his Cardassian lab vanish from the room.