 The Enterprise reaches the Federation colony on the planet Triacus where the adults have committed collective suicide, leaving a recording about an  enemy from within . Yet their orphaned children play seemingly without care or grief, or rather block out the thought of their parents, which they say were always too busy to care for them. In their quarters aboard the Enterprise, the kids conjure a creature they call the Angel which orders them to see the ship goes to the populous planet Marcus 12, and thereto control the starship s crew. Spock finds a scientist s unscientific hypotheses about extreme anxiety striking only the adults and records of an archaeological excavation of the extinct native civilization, which seemingly had one survivor. The children s leader Tommy Starnes disturbs these images, then works his irresistible power of suggestion on the bridge crew, the others on other crew; by the time Kirk and Spock are on to them the Angel is conjured again...