When Voyager encounters a massive energy wave, the ship receives a download of information. The crew tracks the wave's ion trail and finds a stranded vessel with a wounded alien on board. Once the creature is beamed to sickbay, Torres finds that it uses biochemical secretions to give commands. Suddenly, the alien attacks Torres, puncturing her neck and secreting fluids into her bloodstream. Unaware of how to extract the creature without harming Torres, the Doctor and Kim create a hologram of a leading exobiologist named Crell Moset — a Cardassian.
In order to crack the coded message downloaded to Voyager, the databanks from the alien's ship must be accessed. However, the vessel destabilizes and explodes before Seven of Nine can retrieve them. Meanwhile, the Doctor and Crell determine the alien is using Torres as a life preserver by co-opting her vital systems. They re-create Crell's laboratory in a holodeck so he can use his more advanced tools to help Torres, but she objects to putting her life in the hands of a Cardassian.
Unable to decipher the alien's message, Janeway retransmits the signal on all subspace bands hoping more of its species will answer the call. In Crell's laboratory, he and the Doctor operate on a hologram of the alien and find nodes suggesting the creature is a highly intelligent being. Crell decides the nodes are the best place to administer a neurostatic shock, which will incapacitate the alien and probably kill it. Later, the Doctor is shocked when a crewmember, Tabor, reacts violently to Crell's presence on Voyager and calls the Cardassian a mass murderer.
Tabor reveals that during the Bajoran War, Crell used live subjects for his medical experiments and killed hundreds of Bajorans. Although barely hanging on to life, Torres refuses to let Crell treat her. She believes if she benefits from his research, she will be validating his atrocious methods. However, the Doctor cannot remove the alien without Crell's help, and Torres will die otherwise. Against many moral objections, Janeway authorizes Crell to perform the procedure.
As the Doctor and Crell work to extract the creature, Voyager is hailed by one of the alien vessels. They lock the ship in a tractor beam, but Janeway senses they only want their friend back and mean no harm. In surgery, the Doctor overrides Crell's decision to kill the alien and instead administers a neural shock that weakens its motor control without permanent damage. As its tendrils withdraw from Torres, its metabolism is restored and it is beamed to the waiting ship. Left to make a tough decision, the Doctor decides to delete Crell's program from Voyager's database.