Theorizing that one could time-travel within his own lifetime, Dr. Sam Beckett led an elite group of scientists into the desert to develop a top secret project known as Quantum Leap. Pressured to prove his theories or lose funding, Dr. Beckett prematurely stepped into the project accelerator... and vanished. He awoke to find himself in the past, suffering from partial amnesia and facing a mirror image that was not his own. Fortunately, contact with his own time was maintained through brainwave transmissions with Al, the project observer, who appeared in the form of a hologram that only Dr. Beckett can see and hear. Trapped in the past, Dr. Beckett finds himself leaping from life to life, putting things right that once went wrong and hoping each time that his next leap would be the leap home.

Series creator Donald P. Bellisario thought of the idea after hearing the news that marine comrade Lee Harvey Oswald had shot the president, and wondering if he couldn't have done something to stop this from happening. A later episode eventually did broach the subject, but Dr. Beckett was sent to stop Jackie from also dying, rather than sparing President Kennedy.