Si Vis Pacem, Para Bellum

Coming to the aid of another Federation ship, the Discovery is unable to prevent the ship's destruction by a Klingon ship that is using cloaking technology. Desperate for a way to detect these ships even when they are cloaked, Burnham, Tyler, and Saru are sent to Pahvo, a seemingly uninhabited planet with a naturally occurring crystalline transmitter that broadcasts the planet's vibrational frequency into space. They hope to use the transmitter to create a sonar for the hidden Klingon ships. Meanwhile, L'Rell tries to help Cornwell escape in exchange for protection from Kol, but they are caught and L'Rell apparently kills Cornwell to try to save face with Kol. He sentences L'Rell to death for her actions.

Summary from Wikipedia, CC BY-SA 4.0.