A transporter malfunction causes Captain Kirk to split into his "good" and "evil" selves, and a landing party will freeze to death if the crew is unable to find a way to merge the two Kirks back together.
When a mysterious alien race destroys an Earth colony, the Enterprise pursues. However, powerful aliens soon immobilize both ships and force Kirk and the alien captain to fight each other to the death.
The Enterprise narrowly avoids a collision with a black hole and is thrown back to Earth in the 20th century, where they must find a way back and erase any trace of their presence. Matters become complicated when they rescue an Air Force pilot and cannot return him without changing history...but if he disappears that will change history as well.moreless