Delta was a Chimeron queen and, for a time, the last of her species. With the assistance of the Seventh Doctor, Delta was able to revive hopes of survival for her people in face of the Bannermen's genocidal quest against the Chimerons.
Delta fought against the Bannermen when they invaded Chumeria. She was the only survivor. She took a Chimeron egg with her as she fled in a Bannerman ship. She landed at a toll port and boarded a Nostalgia Tours bus headed for Earth, 1959.
The bus landed at Shangri-La camp where the passengers debarked. There, Delta began to fall in love with the resident mechanic and singer, Billy. The egg hatched and she explained to Billy and Melanie Bush that the Bannermen were tracking her down and how she must look after the Chimeron princess.
Billy took her and the child to a spot by a river where they sat for a while; Delta fed the child Chimeron jelly to accelerate her growth. The Seventh Doctor and Ray arrived and warned the pair the Bannermen were coming.
The Doctor took Delta, Billy and the child to stay with Goronwy. Delta caught Billy eating the Chimeron jelly. He told her that, in order to stay with her, he must become a Chimeron. Two Bannermen found their hideout and Delta was able to kill one whilst the Chimeron princess sent them into disarray by singing.
The Chimeron princess used her singing again, though this time amplified to sent all the Bannermen awry, allowing them to be captured. Billy, Delta and the princess then left in their craft along with the Bannermen prisoners, headed for the brood planet. (TV: Delta and the Bannermen)