Captain James T. Kirk of the U.S.S. Enterprise is on an exploration mission in the Prometheus Solar System when the ship comes under attack by an unknown entity. After Mr. Spock takes over for a hysterical science officer Berryman, the enemy is determined to be a Romulan Bird of Prey. The Romulans reject all incoming transmissions but send threats to the Captain. Helmsman Hikaru Sulu fires a barrage of four torpedoes at what appears to be a false image of the Romulan vessel. The vessel becomes critically damaged and the Romulan commander tells Kirk that he was avenging his brother whom Kirk had defeated in a prior engagement. Damage reports show that the ship's protein stock used for food synthesis has been contaminated by leaking phaser coolant. The captain and a landing crew beam down to Prometheus Four, a class M planet with a concentration of life which appears to be peaceful.