On an away mission to an alien planet, Riker is stabbed in the leg by a large thorn on a local vine. He is rushed back to the ships sickbay, but an infection quickly begins to spread through his body. Soon he is unconscious as the infection reaches his nervous system despite Doctor Pulaski's best efforts.
Eventually Pulaski realizes that the infection may be slowed down or even reversed by certain types of neural activity in Rikers brain - activity which is linked to dreaming. She heightens his neural activity, provoking a series of dreams about some of the nice things Riker has done over the last couple of years. Unfortunately, this speeds the spread of the infection up rather than slowing it down.
Pulaski finally realizes that negative emotions must be provoked, and proceeds to do so. Riker experiences various nightmares, which destroy the infection. The ship continues on its way.
Late in the second season of The Next Generation, the powers that be introduced a new menace to our heroes - the Borg. The episode ended with the threat that the Borg would be on their way to the Federation, a trip which would be expected to take several years. However, it was planned that the Borg would actually show up only six episodes later for an exciting season finale. It was a great idea... but unfortunately the show ran out of money and the whole thing was postponed for a year. The planned Borg cliff-hanger episode became "The Best of both Worlds", one of the all time best Trek episodes.
All well and good, but they had to have something for the end of season two, and there was little money left in the kitty. So they resorted to that most hated of all episode formats - the clip show. You'll notice that the plot description above is much briefer than the other best and worst eps I've discussed. This is because the entire plot of "Shades of Grey" is designed to use the minimum possible number of sets, special effects, extras, etc. And of course, most importantly there is a heavy reliance on using clips from earlier episodes. All this makes for a very dull and predictable show.
The clips chosen also show poor thought - interestingly enough, Riker 'remembers' scenes that happened when he was not present! You could say that his imagination was filling in some blanks, but strangely enough what he imagines matches exactly to what actually happened - and what are the odds of that?
Really, I'm not going to say anything more on this one. Some episodes are just so bad that analysing them is like kicking a puppy - you quickly start to feel sorry for the poor thing.