Plot summary
Clara Sutter, a girl who just moved to the Enterprise with her Ensign-father Daniel, is lonely, so she creates an imaginary friend named Isabella to keep her company. One day, to her surprise, her imaginary friend appears, and seems real (though she makes herself invisible before adults see her). Increasingly, Isabella gets Clara into trouble by leading her into off-limit places, prompting Clara to leave her to go play with others. When Clara returns, Isabella is angry and says, "When the others get here, you can die like everyone else."
Counselor Troi, trying to ease Clara's fears of being attacked by Isabella, helps to search the girl's room to ensure she's safe. Isabella appears and blasts Troi with an energy beam from her hand. Later, in sickbay, she relates the event, and Clara works up the courage to tell her father, who then talks to Captain Picard.
The crew learns that Isabella is actually an energy-based life form whose home is the nebula the ship has been exploring. Picard finds Isabella in the arboretum and talks to her about human parenting. Isabella argues that the adults are cruel to Clara and deserve to die. He asks what she bases that conclusion on, and she says it's their treatment of Clara. He asks in what way they've mistreated her, and she lists their refusal to allow her access to certain areas of the ship. Picard explains that rules are for her protection and that Isabella is viewing them through the eyes of a child, which - from that perspective - would definitely seem unfair.
However, they didn't know that Clara's imaginary friend was now real, and was there to protect her. All they knew was that two children were in potentially dangerous areas of the ship, and - as good responsible adults - it was their job to keep them safe. He speaks of how one day Clara will be an adult and she will have rules for her children, and they will be for the same reason. Isabella is convinced, and allows the ship to pass safely through the nebula.
As the ship is preparing to leave the area, Isabella appears to Clara a final time to apologize and ask if they'll ever see each other again. Clara is unsure, but she promises to try and return someday - albeit probably when she is an adult.
Credits
Adapted from Wikipedia contributors, Imaginary Friend (Star Trek: The Next Generation). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.