The stardate for “The Best of Both Worlds, Part I” is 43989.1. It is the opening stardate associated with the episode’s crisis, as the USS Enterprise-D investigates the disappearance of a Federation colony and finds evidence of a Borg attack.
The episode is the third-season finale of Star Trek: The Next Generation and originally aired on June 18, 1990. Starfleet assigns Lieutenant Commander Elizabeth Shelby, a Borg specialist, to help prepare the Enterprise. Admiral J. P. Hanson also arrives as Starfleet confronts the possibility of a full-scale Borg invasion.
The story ends with Captain Jean-Luc Picard captured and assimilated by the Borg, leaving Commander Riker facing an impossible rescue decision. The number is sometimes confused with other nearby TNG stardates, but 43989.1 identifies Part I; the continuation uses a later stardate and completes the cliffhanger in the following season.