TOS: Tomorrow is Yesterday

En route to Starbase 9, the Enterprise is caught in the gravitational pull of a "black star"; when they run the warp engines in full reverse it slingshots them around a star and back in time- to the late 1960's. Air Force Pilot John Christopher is sent to investigate the "UFO" and when the Enterprise attempts to hold him in their tractor beam, it breaks his ship apart, forcing them to beam him aboard. At first Kirk tells him they can't return him to Earth, due to the risk of contamination of the timeline. But Spock's investigations show that one of Christopher's children will make important contributions to the future- and Captain Christopher does not have a son ( yet!) Several beam downs and potential condemnations occur before they devise a way to return everyone to their won time and space- with the future still unknown.