Ace joins the crew in the "Dragonfire" story. She's originally from Earth in the twentieth century, but the Doctor and Mel meet her working as a waitress on the planet Svartos in the far future. This had to do with a time storm created by Fenric. When Mel leaves, the Doctor offers to take Ace with him and she accepts. The TARDIS definitely need a woman's touch, and it's also nitro-9 proof!
How Ace left the doctor isn't known. The series went on "hiatus" after the story "Survival" and didn't resurface for seven years with the Paul McGann movie in which there was no Ace.
Sophie Aldred (born Aug 20,1962). Unlike the nitro-9 toting Ace, Sophie, both before and after Dr Who, has done a lot of work in children's programming. I'm not familiar with BBC children's shows except one about a touchy old guy and a police box, but I'm sure I'd like to see a few episodes of Sophie's "Minnie The Mini Magician." Well done Sophie!