The humanoid inhabitants of Tilonus IV, a world thrown into anarchy by its prime minister's assassination, whose factions captured Commander Riker and tried to strip strategic information from his mind.
The Tilonians appear in the Next Generation episode Frame of Mind. In 2369 their government has collapsed after the killing of the prime minister, and rival groups are fighting for control. Federation researchers are stranded there, and Riker is assigned to go in undercover and bring them out.
He is captured almost at once. His interrogators hold him in a cell and use a neurosomatic technique to dig through his memory. To protect itself, Riker's mind wraps the ordeal in the play he had just been rehearsing aboard the Enterprise-D, in which he plays an inmate of an asylum.
Much of the episode therefore takes place in Ward 47 of an imagined Tilonus Institute for Mental Disorders, with Dr. Syrus, the attendant Mavek and the administrator Suna standing in for his real captors. Riker repeatedly shatters false realities until he truly wakes, overpowers the Tilonians and signals for transport.