This is one of the most brilliant episodes of the series. It's not one of the best, but yes, it's one of the most brilliant. You owe it to yourself to see it if you have any interest in Voyager.
Basically, Seven makes a huge mistake and becomes paranoid to the point where she tries to figure out why exactly she ended up on Voyager, questioning the motives of Chakotay and Janeway, pulling all the things fans assumed the writers had forgotten into a sharp new perspective, and eventually ending in a poignant moment shared between Janeway and Seven in which their relationship is nicely summed up.
No other Star Trek ever did something like this, which is so clever it borders on meta, and does not even feature Q. So many fans tried to claim the writers had no idea what they were doing, or how to tell a good Voyager-specific episode, and then "The Voyager Conspiracy" comes along.
To be clear, if you love the series you'll love this episode. What's the worth of adding a character later on in a show's run? To give some fresh perspective. But this one goes above and beyond the call of duty.
franchise * series * essential * character
Memory Alpha summary.