Agora was a planet located in the Centraxis system.
In the 2150s, the Cybermen invaded Agora and turned it into one of their breeding colonies. After placing human Overseers in charge of conception quotas, they would return every three years to select the five hundred healthiest males for cyber-conversion. In 2175, a rebellion led by Arthur Lakesmith defeated the Overseers, but was swiftly defeated by the Cybermen when they returned the following year. In 2178, six-year-old Grant Markham and three other children were placed on a ship the colonists found on Agora and sent to New Earth. (PROSE: Killing Ground)
In 2191, shortly after meeting the Sixth Doctor and becoming his companion, Grant returned to Agora, where they learned of the Cybermen's rule over the colony. When the Doctor was captured and imprisoned, Grant met with the rebels and aided them in creating the Bronze Knights to defeat the Cybermen. Although reinforcements arrived, the Doctor destroyed these as well, and Agora was finally freed. (PROSE: Killing Ground)
When Lieutenant Kent Michaels met Jamie McCrimmon on Kalaya in 2204, he assumed from his lack of technical knowledge that he came from Agora or some other colony where advanced technology was eschewed. (PROSE: The Final Sanction)