Discovery investigates an ancient Romulan vessel where they face-off with some unexpected guests and uncover a vast mystery that sends them on a race across the galaxy. Meanwhile, Saru is presented with a unique opportunity.

On the heels of their startling discovery, Captain Burnham and the crew investigate a seemingly abandoned planet in their continued quest to solve the galaxy’s greatest mystery. Meanwhile, Saru helps Burnham make an important decision.

On Trill, Captain Burnham, Book, and Culber must pass a dangerous test to prove themselves worthy of the next clue. Adira reconnects with Gray and Saru's first day as ambassador is complicated by his engagement to T'Rina.

On the way to the next clue, the U.S.S. Discovery is sabotaged by a mysterious weapon, leaving Captain Burnham, Rayner, and Stamets as the only crew members who can possibly save the ship in time.

Captain Burnham and Book journey into extradimensional space in search of the next clue to the location of the Progenitors' power. Meanwhile, Rayner navigates his first mission in command of the U.S.S. Discovery, and Culber opens up to Tilly.