Spock builds a 21st computer by assembling vacuum tubes in a series creating a mnemonic memory device. The crude computer reveals that Social Worker Edith Keeler was killed in a hit and run car homicide. At that moment the computer burns up. Spock redesigns and stabilizes, the computer and learns that history has suddenly changed. Edit Keeler does not die because McCoy intervenes and saves her. Edit Keeler leads a growing pacifist movement and becomes the President of the United States. German would win the arms race, destroy Europe, and control the world. The change in world events over the next six years would destroy the Federation space programs causing the Enterprise too cease to exist. Edit Keelers became the focus point in time changing history. McCoy was the random element.
ML - The way the world works - analyzing how things work