Star Trek: The Next Generation - Season 5 Episode 22 -

Young Clara Sutter, like many girls her age, has an imaginary friend. She recently come on board with her father, an engineering officer, and hasn't yet made any real friends. Counselor Troi assures Clara's father that it's perfectly normal and he has nothing to worry about. While playing one day, Clara is shocked to see her friend Isabella suddenly appear before her. In fact, Isabella is a sentient being that has taken human form only to scout the ship and learn more about it. For Picard and the crew, determining the being's intent is key to determining the next step.