It's A Kind Of Magic
Romana was crouching, talking to K9, when the door of This Time Round opened and a young girl stepped in. Romana excused herself from the mechanical dog and asked the child:
"Are you lost?"
"I'm looking for someone. She said she'd be waiting outside, but she isn't."
"Maybe I can help. Who is it you're looking for?"
"Her name is Nyssa."
Nyssa pointed a crossbow at the 11-year-old.
"You're not the Mara I was expecting," she said. "Can I trust you to murder someone?"
Mara Wilson looked horrified.
"No, I suppose not," said Nyssa. "Let's go talk to your agent."