Robin Williams was a massive Star Trek: The Next Generation fan, and the producers of the show even wrote an episode with a part specifically for him – though sadly, he never actually got to appear. Star Trek: The Next Generation was hitting its creative and commercial stride by its fifth season. Beyond being just the most successful first run syndication show of all time, TNG was taking its place as one of the most popular series on all of television at the time. The Next Generation even managed to get Leonard Nimoy to reprise his role as Spock for the two-part episode “Unification,” which was the show’s most-viewed episode up to that point.

Williams had been a American icon since the late 1970s when he starred in the sitcom Mork & Mindy. By the late 80s he’d transitioned to movie star status, headlining such acclaimed films as Good Morning, Vietnam and Dead Poet’s Society. By the early 90s he’d become an A-list box office draw with films like Aladdin and Mrs. Doubtfire.

But just before his film career hit top gear, Williams was approached about guest starring in an episode of Star Trek: The Next Generation. The episode in question, “A Matter Of Time,” would have seen Williams portray a shady time-traveler named Berlinghoff Rasmussen, claiming to be from the 26th century and to have traveled back to the 24th century to monitor an important historical event overseen by Captain Picard and the Enterprise.

In reality, the character was from the 22nd century and simply stole the time machine from an actual 26th century time traveler when he visited the 22nd century. Berlinghoff Rasmussen wasn’t onboard the Enterprise to monitor a historical event; he was there to steal as much 24th century tech as he could and take it back with him to the 22nd century to make himself rich.

Williams ultimately had to drop out of the role due to scheduling conflicts with Steven Spielberg’s Hook. It’s an interesting “what if?” for the franchise, as “A Matter Of Time” aired the week after “Unification” ended. Bringing in a movie star of Williams’ stature could have goosed the show’s already robust ratings to even greater heights. Williams was replaced by Matt Frewer, who does a more than serviceable job as Rasmussen, but there’s no denying the episode lacked the magic it would have had if Williams had been able to play the part. It’s one of the bigger missed opportunities in Star Trek: The Next Generation’s history, and it’s a genuine shame we never got to see Robin Williams in the final frontier.