Clearly, this episode is one of the best of Millennium. But, unlike what you expect, it does not pursue the main story of Millennium - the fight between the good and the evil, the end of the world, and a group of people who want to control, etc.
If you think this episode is just a story about a case that occured fifty years ago at a highly-secured research facility, a suicided former-agent and a conspiracy related to that, watch this again. Watch between the lines.
"IT MUST END." It's not just the dead agent's message but also the creators of Millennium's. Humankind made a evil fifty years ago, and now, we are about to make another evil. They tell us, "stop playing god!" This is why I like the series. This is why I like Chris Carter.
Matryoshka… Now, whenever I hear it or see it, I remind this episode and their message. Of course, "Till Then" became one of my favorite.