The operation of 3189 in which the Emerald Chain leader Osyraa captured the USS Discovery, used it to get inside Federation Headquarters and tried to force an alliance on Admiral Vance.
The Emerald Chain attack on Federation Headquarters runs through the Discovery episodes Su'Kal, There Is a Tide and That Hope Is You, Part 2. While Saru and others are away in the Verubin Nebula, Osyraa's flagship Viridian seizes Discovery and its acting captain Sylvia Tilly.
Osyraa stages a pursuit so that the damaged ship is let through the headquarters shield. Once inside she surprises Vance by proposing an armistice and a merger of the Chain with the Federation. He is interested, but he asks that she stand trial for her crimes, and she walks out.
Michael Burnham and Cleveland Booker slip aboard, and Burnham sends Stamets off the ship so that he cannot be made to jump it. Tilly's bridge crew, helped by the ship's repair robots, win back control. Burnham kills Osyraa, the Viridian is destroyed, and Booker discovers that he can pilot the spore drive.