Star Trek: Lower Decks is more serialized than other shows set during its 24th Century era, save the last few seasons of Star Trek: Deep Space Nine. One element this season centered on a mysterious vessel capturing crews of Ferengi, Klingons, Romulans, Orions and even the fast-talking Bynar. In the penultimate Season 4 episode, Lower Decks reveals that a deep-cut character from Star Trek: The Next Generation is behind it all: Nicholas Locarno. This episode and the finale essentially serve as a two-parter, much like the series did to end Seasons 2 and 3.
Source：https://www.cbr.com/star-trek-lower-decks-answer-season4-mystery/
