Colonel Fellini
Ed Peck

Technician Webb
Richard Merrifield

Captain John Christopher
Roger Perry

Military Police Sergeant
Jim Spencer

Transporter Chief Kyle
John Winston

While en route to Starbase 9 for re-supply the Enterprise encounters the peculiarly strong gravity well of a black star. By applying maximum power from the warp engines the ship is able to break free but is thrown away from the phenomenon at an incredible speed, in a 'slingshot' effect, as the gravitational pull is broken. The ships systems go down under the strain, and, after effecting repairs to restore auxiliary power, they find themselves in a low orbit around Earth.

Kirk orders Uhura to contact Starfleet to advise them of the situation. They manage to climb high enough to attain a safe orbit but Starfleet cannot be reached. Instead, Uhura is picking up old-fashioned radio broadcasts about the first lunar landings. The Enterprise, it seems, has been thrown back in time to the late sixties.

While in low orbit the Enterprise was detected by U.S. Air Force radar and an aircraft, piloted by Captain John Christopher, has been sent to investigate. The plane closes to visual range and Kirk orders the use of the tractor beam to hold it at a distance but, unable to take the stress, the 'primitive' aircraft starts to break up. Kirk is left with no choice but to save the pilot by beaming him aboard. Kirk meets Christopher in the transporter room and explains the situation to him. By now power has been restored to the deflector shields rendering the Enterprise invisible to ground radar but the damage has already been done and their time line is already in danger due to Christopher's 'view of the future'.

Following a meeting, at which Spock announces that Christopher made no significant contributions to history, they plan to keep him aboard but Christopher has other ideas. He has a wife and children back on Earth and also feels a strong duty to report what he has seen. With this in mind, he manages to escape and gets as far as the transporter room where Spock and Kirk manage to stop him. Spock has been making further enquiries in the history databanks and has discovered that, although Christopher himself made no impact on history, he has an, as yet, unborn son to be called Sean Jeffrey Christopher who is to lead the first successful Saturn expedition, and so must now be returned.

Kirk and Spock beam down to the Air Force base in order to remove any recorded references to their appearance. Various events occur, including the momentary beaming up of a military policeman by mistake, but their mission is a success. Their only problem now is to somehow return Captain Christopher to Earth. Scotty and Spock make a detailed examination of the events leading to their current situation and come up with some calculations that will allow them to perform the manoeuvre in reverse by using the gravitational pull of the sun.

They theorise that, if transported at exactly the right moment on the return 'trip', Christopher can be returned to a moment in time shortly before he got his first glimpse of the Enterprise. Christopher returns to the transport room while Spock makes the necessary arrangements. At first the Enterprise travels further back in time, but as the 'slingshot' effect takes over she is thrown forward into the future at a fantastic rate.

The incredibly accurate transportation of Christopher is successful. The Enterprise goes through incredible stresses, braking heavily at just the right moment to end up in her own time again. The engines start to give way and the whole ship shakes violently but she holds together and, as they slow to 'normal' speeds, Uhura receives an incoming message from Starfleet meaning that they are back 'home'.