Si Vis Pacem, Para Bellum

After failing to save the USS Gagarin from cloaked Klingon ships, Starfleet sends Burnham, Saru and Tyler to Pahvo, a planet whose crystal structure broadcasts its natural vibrations into space. The plan is to turn it into a sonar that can detect cloaked vessels. The team discovers that Pahvo is home to ethereal beings who live in harmony. For the first time in his life, Saru is free of the fear that defines his species, and he wrecks the team's communicators so they can never leave. Burnham escapes, fights him off and sends the modified signal. Meanwhile, on Kol's flagship, L'Rell secretly offers to help the captive Cornwell escape, but when they are caught she claims to have killed her, and Kol condemns her to death. On Discovery, Stamets hides worrying side effects from the spore drive. In the end the Pahvans, wanting to bring both sides to peace, send their own signal to the Klingons, who head for Pahvo.