Stardate 1312.4: On the way to the Galactic Barrier the Enterprise picks up a disaster recorder from the old Earth ship S.S. Valiant that disappeared 200 years ago, indicating that something related to extrasensory perception (ESP) happened to the crew. The Enterprise crosses the barrier nonetheless, whereby nine crew members are killed and the engines burn out. Moreover Kirk's old friend Gary Mitchell begins to develop ESP powers that pose a growing threat to the ship. Kirk assigns psychiatrist Dr. Elizabeth Dehner to observe him. When the Enterprise arrives at the Delta Vega station for repairs, Kirk sees no other way but to exile the dangerous individual that Mitchell has become on the uninhabited planet, with the option to blow up the facility if necessary. Mitchell, however, kills Lt. Kelso and escapes with Dehner who too has mutated in the meantime. Kirk goes after them with a phaser rifle, but Mitchell easily anticipates the attack and prepares to execute his old friend. Dehner, with what is left of her humanity, decides to help the captain and weakens Mitchell at the expense of her own life so Kirk can kill him.

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