In the pilot episode, Dr. Samuel "Sam" Beckett is seen prematurely beginning some sort of experiment by stepping into a fictional scientific apparatus called the quantum accelerator, where he vanishes. He awakes with a sort of amnesia, his damaged psyche trapped in the body of a fictional Air Force pilot in 1956. In each episode, Sam "leaps" into another character, and is required by whatever force controls his leaps, assumed to be the Lord, to put something right that once went wrong sometime in the past.
Dr. Sam Beckett Sam himself is the protagonist, a prudish and benevolent Christian willing to put right as many things as he must in order to leap home. He was born in Elk Ridge, Indiana, in August 1953, with an older brother Tom and younger sister Katie. He holds six doctorate degrees, including one in quantum physics, medicine, and ancient languages.
Admiral Al Calavicci Al is Sam's friend, who appears as a hologram only visible to Sam ... and to babies, who cannot be trained to pretend not to see actor Dean Stockwell ... and to young children, who play important roles when seeing Al in certain leaps ... and to the mentally ill ... and to a random Scrooge equivalent in an episode based on Dickens' A Christmas Carol. He has had six wives and often takes advantage of his holographic form to seek naked women, a likely resulting of his first wife having abandoned him while he was MIA in the Vietnam War.
Ziggy Ziggy is a computer with an ego, capable of providing data to Al the hologram, which he passes on to Sam, as well as making predictions as to what Sam must do in order to leap.
Sam theorized that he should be incapable of leaping outside his own lifetime - however, he leaps into April 1953 in the final episode of the first season, and into the Civil War near the end of the fifth season. Some of Sam's most famous leaps include:
- Leaping into the mentally retarded Jimmy LaMotta, in a fan favorite story
- Leaping into a police office who is in San Diego with Al's first wife, Beth, while Al is MIA
- Leaping into himself as a 12-year-old boy
- Leaping into a man about to be near-fatally electrocuted by an angry shock therapist
- Leaping into the future while Al is stuck in 1945
- Leaping into a young Al Calavicci
- Leaping into a man with PTSD and feeling the symptoms
- Leaping into Lee Harvey Oswald and feeling the urge to kill JFK
- Leaping into Elvis Presley
- Leaping into a "hurricane party" in 1969 before the landfall of Hurricane Camille
- Appearing in the past as his future self, with his graying hair and new wrinkles, at the exact moment of his birth
It is determined during the leaping that Sam, while appearing as Scott Bakula to the camera, will appear as someone else to other people, in mirrors, in photographs, or on camera. When Sam leaps into a blind person, retarded person, or handicapped person, he can still see, think, and walk. When Sam leaps into anyone, that person appears in the quantum accelerator, later called the waiting room, wearing Sam's clothes while he wears his or hers.