Harry Mudd uses a time crystal to loop the same half hour aboard Discovery so he can sell the ship to the Klingons, and only Stamets notices.
During a crew party, Discovery takes in an endangered space creature called a gormagander, and Harry Mudd emerges from it. Using a time crystal, he resets time over and over, learning the ship's secrets and killing Lorca repeatedly so he can sell Discovery to the Klingons.
Because of the tardigrade DNA in his body, Stamets remembers each loop. He convinces Burnham, and with Tyler they work through many repetitions to outwit Mudd, who is finally handed over to his wife Stella and her father. Burnham and Tyler grow closer during one of the loops. The episode was written by Aron Eli Coleite and Jesse Alexander and directed by David M. Barrett.