This case file predates the creation of the Temporal Investigation Agency by several years, and so there is limited information available. The logs of the Starship Enterprise NCC 1701 indicate that whilst the ship investigating unusual temporal phenomena the ships Doctor, Leonard McCoy, was accidentally injected with a massive overdose of cordrazine. Suffering from paranoid delusions as a result, he beamed down to the alien planet which was the source of the phenomena. The landing party which persued him discovered what appeared to be an ancient artefact capable of transporting objects through large amounts of both time and space. The artefact appeared to be sentient; in response to their voices it explained that it had been waiting for "a question... since before your sun burned hot in space and before your race was born" - a period of at least five billion years. The artefact called itself the Guardian of Forever, a gateway to other times and places.

Whilst the landing party was distracted McCoy entered the device and was transported to 1930's Earth. His actions there created an alternate timeline which differed from the preferred one in many respects; most notably, the Federation was never created in this timeline. The landing party was isolated from these changes, apparently as a result of their proximity to the artefact.

The Enterprise Captain, James T. Kirk, and the science officer Spock followed McCoy, emerging some weeks before him. They were able to use tricorder records of the Guardian to discover the key event which McCoy had altered - he had saved the life of a woman named Edith Keeler, who had gone on to prevent US entry into World War II. Germany thus developed atomic weapons first and conquered the world with them. In the preferred timeline the war was followed by decades of fierce international technological competition which, amongst other things, resulted in the development of space flight. Temporal scanning has shown that in the altered timeline the world-spanning German Empire lacked any such motivations, and so never pushed out into space.

Although Kirk's logs do not describe his time on Earth in any detail, it is widely rumoured that he developed considerable personal feelings for Edith Keeler. Nevertheless, when he located McCoy he prevented him from saving her life.

The very presence of the three officers in the past would normally mean that the original timeline would not be perfectly restored. Temporal scanning has proved that at least one other resident of Earth died as a direct result of McCoys arrival, when he stole the Doctors phaser and accidentally activated it. Many other minor changes would also have been introduced as a matter of course. However, the Guradian indicated to Kirk that all aspects of the original timeline had been restored after his arrival. Detailed temporal scanning has indicated that the events of this trip into the past had an overall zero impact - something that is normally conisdered impossible. There is as yet no satisfactory explanation for this, although many suspect that the Guardian itself could have somehow "repaired" the timeline.

Considerable study has been made of the Guardian of Forever over the last century. Unfortunately, the object still defies all analysis. Despite decades of exhaustive scanning no mechanisms have ever been discovered within the Guardians structure; it appears to have nothing remotely similar to information processing systems or moving parts, nor any kind of any generation system - although it must be able to generate and control massive amounts of energy in order to do what it does. The Guardian itself is quite willing to converse with researchers, but maintains that its operating principles are beyond our comprehension.