A strange beam shrinks the Enterprise crew, and its source proves to be a tiny lost Earth colony on a volcanic planet that needs rescuing.
While studying a burnt-out supernova, the Enterprise picks up a message in a two-centuries-old code and is hit by a beam of spiroid radiation that damages its dilithium and shrinks the crew. As they grow smaller the bridge crew vanishes, and Kirk, alone and tiny, finds that the transporter can restore people to normal size.
The beam came from Terratin, a lost Earth colony once called Terra Ten, whose people were reduced to a minute scale by radiation over generations. Their world is wracked by volcanic activity, and the beam was their way of getting attention. Kirk restores his crew, then beams the whole miniature city aboard and moves it to a safer planet. Paul Schneider wrote the episode.