According to Deadline, both Anson Mount and Rebecca Romijn won't be returning to the Trek prequel after the second season. Mount, who played Captain Christopher Pike, and Romijn, who played Number One, were brought into the Trek prequel specifically to help bring Discovery's storyline closer to the original series, and were only given one-season contracts.
Both characters appeared in Star Trek's earliest episodes, which are set roughly 10 years after the events in Discovery. Pike, of course, was captain of the USS Enterprise before William Shatner's James T. Kirk took over the chair.
Pike's tragic fate was addressed on the original Star Trek series. While no details have been revealed on how the two characters will exit Discovery, Mount himself told SYFY WIRE back in January that his storyline won't be ending in tragedy.
Deadline also notes that Mount and an unnamed Discovery director got into an altercation on set, as previously reported. That issue was resolved by CBS's HR department, and apparently didn't factor into this decision.
This isn't the only casting change that the show will be dealing with. Over the weekend at Emerald City Comic Con, Rainn Wilson also confirmed that Harry Mudd won't be returning for the show's current sophomore season, but could still potentially return for round three, which was just confirmed last month.
Star Trek: Discovery is nine episodes through Season 2 now, with five more to go. You can check out new episodes every Thursday via CBS All Access.