Star Trek: 25 Greatest Next Generation Episodes

21. Parallels

In what is probably the second closest thing to a mirror universe episode in the series (sans possibly "Yesterday's Enterprise"), Worf returns from a victorious bat'leth competition on Kronos and he starts to notice subtle differences in the continuity of events at random times as little as objects and people in a room and as big as Captain Picard is dead, he is Riker's first officer and Wesley is tactical officer.

The big problem is, though, that Worf is believed to have suffered a concussion during his competition on Kronos so his hair brained theories are originally dismissed until Troi randomly starts walking into his quarters and saying she's his wife. One of the more horrifying parts of this episodes involved hundreds of thousands of Enterprises coming through pockets in space from other universes.

Data touches on the theory that anything that can happen has actually happened in alternate realities. One of these alternate realities includes a universe where the Borg have assimilated the entire Federation and the Enterprise and her Riker, sporting a rather hilarious Jesus-like beard, opens fire on a shuttle carrying Worf to our indigenous universe, refusing to return.