The Chimerons were being persecuted to the point of extinction by the Bannermen. Two Chimerons, a male and the Queen, Delta, managed to steal a Bannerman craft. The male was shot, but Delta, along with an unhatched egg, managed to make it to Earth in 1959.
A bounty hunter on Earth gave away the Delta's location, and the Bannermen journeyed to Earth intent on the genocide of the Chimerons. Whilst on Earth the egg hatched, giving rise to another Chimeron Princess.
The seventh Doctor and Mel helped Delta and her baby, who grew very quickly, to defeat the Bannermen. Billy, a mechanic at a holiday camp, fell in love with Delta and decided to ingest the baby's food in the hope that he would become a Chimeron. The plan worked, and together Billy, Delta, and the Chimeron Princess left together to rebuild the Chimeron race and to take the Bannermen to face justice.