In 2270, Breen was part of a landing party that explored the surface of a previously unexplored planet near Starbase 11 and discovered ruins of a decadent city full of pleasure palaces, along with James T. Kirk, Spock, Leonard McCoy, Nyota Uhura and Brad Collins. Exposure to alien machinery swapped the sexes of all but Spock, changing Breen into a burly tall man with broad bones. Breen rescued Collins from a handling robot.
After the rest of the ship's crew had been processed, and the ship escaped to Starbase 11, Kirk considered returning, believing that the machinery would not recognize them and would re-swap everyone's sexes. Breen realized that he'd become the most male among the crew, and as a result was most likely not be switched back, but to be given an extra male chromosome, as had happened with Spock on their first visit. He preferred to remain a man, but was willing to return.