The Vulcan Hello

Investigating a damaged satellite near a binary star system on the edge of Federation space, the crew of the Starfleet starship USS Shenzhou discover an object obscured from their sensors. When First Officer Michael Burnham volunteers to investigate the object, she finds an ancient, carved vessel. She is attacked by a Klingon, and when trying to escape, she accidentally kills him. A group of Klingons mourn the death of their soldier, dubbed the "Torchbearer", before an outcast Klingon named Voq volunteers to take his place. The Klingons, led by T'Kuvma, reveal themselves to be in a cloakable ship. T'Kuvma preaches to his followers about the Federation's attempts to usurp the individuality of the Klingons and their culture, and plans to fulfill an ancient prophecy by uniting the 24 great Klingon houses as was once done by the legendary Kahless.

Summary from Wikipedia, CC BY-SA 4.0.