B'Elanna is on a planet where violent thoughts are a crime.
Voyager visits the homeworld of the Mari, a telepathic people who have eliminated violence by policing even violent thoughts. After a man bumps into Torres and she has a flash of anger, the same man brutally assaults someone, and the Mari authorities conclude that her thought triggered the attack. She is sentenced to have the memory removed, a procedure that could damage her brain.
Tuvok works with chief examiner Nimira and discovers that a Mari merchant is secretly trading in illicit violent thoughts, which he had extracted from Torres and passed on, causing further attacks. Torres is freed just before the procedure, and Nimira is forced to accept that her society has not truly rid itself of violence.