Quantum Leap was a science fiction television series which ran from 1989 to 1993 on NBC. It followed the adventures of Dr. Sam Beckett (played by Scott Bakula), a brilliant scientist who finds himself abruptly and uncontrollably 'leaping' through time, temporarily switching places with diverse people at various times within his own lifetime, the second half of the 20th century: "leaping from life to life, striving to put right what once went wrong, and hoping each time that his next leap... will be the leap home".

The show's ninety-seven episodes aired on NBC between March 1989 and May 1993.

Dr. Sam Beckett (born August 8th, 1953) is a brilliant theoretical physicist with six doctoral degrees in subjects including quantum physics, medicine, archeology, music, and ancient languages. (For the record, we know he doesn't have advanced degrees in literature, psychiatry, or the law). Dr. Beckett theorizes that a person may be able to time travel within the confines of his own lifetime. In the near future, at a highly classified U.S.-government-funded research facility somewhere in the desert of New Mexico, Sam is working on a grand experiment to prove his theory. However, the funding for Project: Quantum Leap is about to be cut. In an effort to prove that his theories are correct, Sam steps into the project's "accelerator chamber" and vanishes.

Sam appears in the past with no memory of who he is or where he is. This side-effect of uneven amnesia is called Swiss-cheesing or (as a technical term in the show's universe) magnafluxing, which prevents him from remembering most of the details of his own life. His friend from his original time, Albert "Al" Calavicci (played by Dean Stockwell), appears to him as a holographic projection from the "imaging chamber" — usually only visible and audible to Sam. Al is the project observer and a U.S. Navy Rear Admiral. Along with the (possibly) sentient supercomputer named Ziggy, Al is able to help Sam "set right what once went wrong" before he leaps out in to the next person. At the beginning and end of nearly every episode, as Sam leaps into a new person, the catch phrase of "Oh boy..." was uttered.

In the pilot episode, Sam has lept to the year 1956 as an X-2 test pilot, Captain Tom Stratton. In one of his holographic visits, Al tells Sam about Ziggy's theory that "God, or Time, was just waiting for your quantum leap to... correct a mistake." Al thinks that this is "a load of crap", but "If Ziggy's right, all you have to do is break Mach 3 and live." (Al also suggests that he wait 40 years and Sam will be in "the present.") But as more of these seemingly random leaps put Sam in a position to fix something that once went wrong, Al gradually comes to believe that the experiment has been mysteriously co-opted by an unidentified higher power, to use Sam to avert tragedies in ordinary people's lives. This is later confirmed indirectly when Sam meets an "evil leaper" who knows that her job is to set wrong what once went right.

The term holographic projection is used in the program, although it is not the same as real holography. The show's "hologram" is a three dimensional, neurological projection; "created by an agitation of subatomic carbon quarks tuned to the mesons of my optic and otic neurons." To project the hologram, Al enters an "Imaging Chamber" in which the image of Al and anything he is touching, e.g., a person or cigar, are visible to Sam and Sam can hear Al speak, and correspondingly events in the past are visible and audible to Al. However, throughout the series, it has been found that animals, young children, and the mentally ill can see Al. This has been used to Sam's advantage on a few occasions, such as Al soothing a crying child, leading a dog away from Sam, or speaking directly with an asylum inmate.

In what may be a form of paradox, in one episode Sam ends up leaping into Al himself at an earlier period, when Al is on trial for murder. Part way through the episode, when it appears that the case is going badly against Al, Dean Stockwell disappears mid-sentence and is replaced by a character played by Roddy McDowall, implying that Al was convicted and executed.

The Quantum Leap generator is run by a supercomputer lovingly called Ziggy which can use its immense database to pinpoint where and who Sam is and help Al figure out why he is there and what he must do so everything can be put right. Ziggy (originally referred to as a "he" and then later a "she") almost always had a specific timeline to accomplish the objective like 2 days, 5 hours, 10 minutes, and 50 seconds. Almost every episode centered around what Ziggy is trying to tell Sam to do, and giving him a clear objective, like making sure someone doesn't end up in a car that will crash, saving a child's life or having someone stand up for themselves after an attack like a rape or hate crime. Almost always, what Ziggy said was confusing and left Sam and Al to figure out in the last minute what had to be done so everything would be put right and Sam could leap.

- From Wikipedia