Worf slips between alternate realities, including one where the Borg won.
On his way back from a bat'leth tournament, Worf finds that small details around him keep changing. He is in fact shifting between parallel realities, in one of which he is married to Troi.
Geordi's visor and a quantum fissure in space are behind the shifts. When the Enterprise tries to send Worf back, many versions of the Enterprise from different realities appear at once, including one from a reality where the Borg have overwhelmed the Federation. Worf finally returns to his own reality in his shuttle, and the other ships disappear.