The very first "Star Trek: The Next Generation" episode, "Encounter at Farpoint" wasn't particularly good. The show's first meeting with Q was, to put it bluntly, pretty dull stuff. Fortunately, the more times this supreme being appeared in the show, the better these meetings became--and "Déjà Q" is exceptional.
When the show begins, Q makes his most memorable entrance ever! While the ship is trying desperately to save a planet below, Q arrives and expects everyone to drop everything and welcome him. As usual, however, he's made to feel about as welcome as an outbreak of herpes! In an odd twist, Q is now mortal--turned into a human by the Collective. Why exactly is uncertain--all you know is that Q is very whiny and hates the prospect of living out life this way.
Although the Q plot is diverting, the crew really must work on stopping the moon from leaving its orbit and destroying the planet, so much of the time Q whines and everyone but Data ignores him. As for Data, his conversations with Q are awfully funny. Bizarrely, despite Q's immense knowledge, no one considers asking him to help save the planet. Still, despite this, Q does manage to save the day! How and why? Well, see the show.
This episode has a lot of fun moments and just goes to show you that the humans on the show are pretty dumb and NEVER are the least bit grateful for his help--and he IS quite helpful here despite himself. Well worth seeing and memorable...and a sign of continual improvement in the Q episodes.