s2e16 “Q Who”
For some context: After the hot chocolate incident, Picard enters a turbolift to change his uniform and exits into a shuttle with Q. Long story short, he introduces the Enterprise to the oncoming threat of the Borg… yadda yadda yadda… 30ish years later he stops them “for good.”
With a LOT of help from Janeway.
I said yadda yadda in the notes. She did do a lot of the heavy lifting, in all fairness.