A Chimeron queen called Delta, the last surviving member of her race, is being pursued by the evil Gavrok and his Bannermen, intent on a mission of genocide. Delta finds herself on board a space bus of tourists en route to Earth when it is knocked off course by an American satellite and ends up at a Welsh holiday camp in 1959.

The Doctor and Mel, having won a holiday with the tourists, help Delta to evade the rampaging Bannermen long enough to allow her child to hatch from its egg and grow to maturity. She, her daughter and a young man named Billy - who sacrifices his humanity to be with her - then escape to start a new life and ensure the continuation of the Chimeron race.