A pair of scientists warn that warp travel is damaging space.
The Enterprise is investigating the disappearance of several ships in a region of space, and a pair of scientists claim that the cause is the warp drive itself, which is damaging the fabric of space. The Federation is skeptical, but the evidence builds. Soon, the Enterprise itself is in danger.
The story presents a serious ecological message, in which the very technology that has made exploration possible threatens the environment. The result is a new speed limit for warp travel. The episode is a thoughtful and somewhat controversial one, with a message about the responsibility that comes with progress.