Trelane, the imperious, godlike figure Kirk's crew once dismissed as a spoiled child playing with power, turns out to be Q's actual protégé, and when he finally masters the Continuum's power he starts tearing across timelines instead of star systems. Picard's Enterprise gets caught in three colliding realities, one where Jack Crusher never died and commands the ship himself, forcing Q to rein in his wayward student before the multiverse comes apart at the seams.
Summary by Loreful, from public sources