While visiting the world of the Mari, a telepathic race, a Mari man named Frane collides with Torres in the marketplace. Soon after, Frane viciously attacks another man. During questioning, Torres admits to Chief Examiner Nimira that she had thoughts of hurting Frane when he bumped into her. To her surprise, Torres is arrested for "aggravated violent thought resulting in grave bodily harm."

Torres is told she must undergo an engramatic purge, a medical procedure to identify and remove the offending images from her mind. Frane will go through the same purge. Janeway protests when she hears there's a risk of neurological damage, and she sets out with Tuvok to review the evidence. They find that Frane has a record of previous arrests for harboring violent thoughts, but Nimira claims he has been cured through previous purging. In the marketplace, a Mari woman experiences Torres' violent thought against Frane and attacks a young merchant, killing her.

Puzzled by the two related attacks among these normally non-violent people, Tuvok mind-melds with Torres to learn more of her brush with Frane. He learns that a Mari named Guill approached her after the incident and probed her mind. When Tuvok questions him, Guill telepathically senses the Vulcan's dark impulses and offers to help Tuvok control them. After witnessing Guill secretly accept money from another Mari, Tuvok realizes Guill may have a business interest in violent thoughts. He approaches Guill again, offering to telepathically exchange his disturbing images.

Questioning him further, Tuvok learns Guill buys violent thoughts from people or sometimes takes them, as he did with Torres. Guill denies he's responsible for the marketplace attack or for the circulation of Torres' thoughts, but Tuvok wants to take him to Nimira for questioning. Just then, two of Guill's conspirators intercept them. Meanwhile, Torres begins the engramatic purging.

Tuvok lures Guill into a mind-meld with the promise of transferring his violent thoughts, but instead he inflicts mental pain on Guill and brings him to Voyager. Janeway gets Nimira to stop the purging procedure after Tuvok explains that Guill and Frane conspired to provoke the thought in Torres.