The person inside the beam is Gary Seven, a guy with a fancy suit, a cute cat, and a mission: he is a supervisor sent through time and space on behalf of an unnamed organization to check on some missing agents.
While visiting the past, he ends up detonating a missile, an action he insists averted World War III. We never find out more about who Gary Seven works for, mostly because this was meant to be a backdoor pilot for a new series that never materialized.