"In a Mirror, Darkly" is the eighteenth and nineteenth episodes of the fourth season of the American science fiction television series Star Trek: Enterprise, and originally aired on April 22 and 29, 2005. This installment was developed to be a sequel to The Original Series episode "The Tholian Web" and a prequel to "Mirror, Mirror". The decision to set an Enterprise episode in the mirror universe originated with a pitch to enable William Shatner to appear in the series. The teleplays for both parts of the episode were written by Mike Sussman, with Manny Coto contributing the story for the second part.

The fourth season of the American science fiction television series Star Trek: Enterprise commenced airing on the UPN network in the United States on October 8, 2004 and concluded on May 13, 2005 after airing 22 episodes. Set in the 22nd century, the series follows the adventures of the first Starfleet starship Enterprise, registration NX-01. The fourth season saw changes made to the production team, with Manny Coto becoming the show runner. He had joined the team during the third season as a co-executive producer. Other changes included Star Trek novelists Judith and Garfield Reeves-Stevens being brought on board as writers.