Stardate 1512.2: During a routine star-mapping assignment in an uncharted section of the galaxy, the Enterprise encounters a space bouy of unknown origin. At first the bouy blocks the ship's path, but when Kirk decides to maneuver around the bouy it charges the ship, forcing Kirk to destroy it with phasers. After consideration, Kirk decides to continue on in an effort to discover the intelligence behind the bouy. Soon the Enterprise is confronted by a gigantic alien vessel, the Fesarius, which promptly grabs the Enterprise with its tractor beam and scans its record banks.

After a futile effort by Kirk to communicate with the alien ship, Balok, the commander of the Fesarius, decides that the Enterprise must be destroyed and gives the crew 10 minutes to prepare. Lt. Bailey, a young and inexperienced bridge navigator, cracks under the pressure and is relieved from duty. With only a few minutes to spare, Kirk tries to bluff Balok, telling him that Earth ships are equipped with a corbomite device which, when touched with destructive energy, has the power to destroy the attacking vessel. Balok balks and decides instead to tow the Enterprise with a small pilot ship to an area where the crew will disembark and the vessel can be destroyed.

While being towed, the bridge crew monitors Balok's ship, hoping he will grow careless. After Balok pulls ahead a bit and lowers his power levels, the Enterprise breaks Balok's tractor beam. Balok sends a distress call to the Fesarius, but it is too weak to be heard. Despite the danger, Kirk, McCoy, and Bailey beam over to the pilot ship and discover that the Fesarius is manned by one small and very friendly alien who has been testing the crew of the Enterprise to discover their real intentions. After deciding that an exchange of information would benefit both cultures, Bailey volunteers to stay behind as the first envoy to the First Federation.