An Earth-like planet was lush and tropical. It was home to an advanced humanoid race. The black-haired people were the masters and the fair-haired people were the slaves.
When the Second Doctor arrived on the planet, he was greeted by Tonga and his people, who assumed that Ben Jackson and Polly Wright were the Doctor's slaves. They were taken to the leader of the black-haired people, who argued that the slaves were treated well, but said that they would be freed if they could pass the two tests of Trefus. With the Doctor's help, the fair-haired people passed the tests and were freed. (COMIC: The Tests of Trefus)