Best of Both Worlds Part 1 & 2
Jonathan Frakes was so popular that the producers of Star Trek: The Next Generation had set up his character of Will Riker as the next face of the franchise come the end of season three. Patrick Stewart was waffling about returning for a fourth season, and the writers set up the end of the season, where Picard gets abducted by the Borg, as a way to write Picard off the series should he leave. Due to the idea behind the episode basically serving as a way to put Rider in the captain’s chair, the entire two-part episode featured Frakes’ Riker carrying the load. The end of the first part of the event, saw Riker order the Enterprise to fire on Picard, and the drama of that moment before it went to black was among the best instances of such an event in franchise history.
Not to be confused with the film, the episode First Contact has Riker getting surgically altered to fit in on a planet the Enterprise is exploring. The species is nearing warp travel technology and Riker and company wanted to explore the planet undetected. It’s an episode where humanity ended up being the visiting aliens that got discovered by the people who live there.