Trials and Tribble-ations (Star Trek: Deep Space Nine, Season 5, Episode 6)
Remember the tribble episode from the original series? It was so beloved that DS9 gave it a sequel.
In the episode, the DS9 crew has to travel through time to prevent a Klingon from murdering Captain Kirk with a booby-trapped tribble. The show used digital imaging to insert characters into the original tribbles episode and recreated the sets and costumes from the original series. The episode was nominated for three Emmys and brought back a number of original cast members both onscreen and behind the scenes.
Watch Star Trek: Deep Space Nine on CBS All-Access.