Plot summary
During training for a covert mission, Riker is accidentally injured by Lt. Worf, and while Dr. Crusher heals the head wound, some pain persists. The next day, Riker performs in Frame of Mind, a play in which his character is confined to a mental asylum, and delivers a soliloquy regarding the nature of being sane. At the play's conclusion, one officer frowns, ignoring the ovation the audience delivers. As Riker straightens from a bow, he is alone, trapped in a cell similar to the set. As he is locked in the cell, Riker hears an alien humanoid doctor deliver a line from the play.
In the asylum cafeteria, Riker becomes agitated when he is reminded that he killed a man, prompting doctors to sedate him. Awakening on the Enterprise, Riker flees from an alien doctor only to find himself back in the asylum cell, where medics use holographic projections of the Enterprise crew to convince him that the asylum is the true reality. This succeeds in convincing Riker to ignore what he believes is a hallucination of Dr. Crusher, who tries to alert him to an impending rescue attempt, which succeeds in returning him to the Enterprise - which Riker remains convinced is a hallucination.
To prove this, he fires a phaser at himself, revealing that he remains in the asylum cell. In a bid to destroy the cell he sets the weapon to overload, and after it detonates, finds himself back on the theatre stage taking an ovation. Refusing to accept this as real, Riker pounds on the wall of the set, shattering that reality.
Finally coming to on an operating table, with a device inserted into his head where he has been experiencing pain, Riker struggles free and uses his communication badge to request an immediate beam-out. Back aboard the Enterprise, Dr. Crusher tends to his wounds, and Riker learns that he had been taken prisoner on the covert mission. The strange experiences had been prompted by his resistance to efforts to scan his brain to discover strategic information about the Federation. Riker returns to the set of the play one last time to dismantle it.
Credits
Adapted from Wikipedia contributors, Frame of Mind (Star Trek: The Next Generation). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.