The Omega Directive

Voyager is forced out of warp when it detects the most powerful substance known to exist called "Omega," which only Janeway and Seven of Nine know about onboard the ship. The Borg experimented with "Omega" in the past. Due to the nature of the particle, a top-secret Starfleet directive gives knowledge of it only to Starfleet Captains and orders its destruction at all costs because the unstable molecule has the power to destroy subspace.

Synopsis adapted from TVmaze (CC BY-SA 4.0)