Scientist Sam Beckett finds himself trapped in time--"leaping" into the body of a different person in a different time period each week.

[Repeated line]
Sam: Oh, boy...
[repeated line - season 2]
Narrator: Theorizing that one could time travel within his own lifetime, Doctor 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, Doctor 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 made through brainwave transmissions, with Al, the Project Observer, who appeared in the form of a hologram that only Doctor Beckett could see and hear. Trapped in the past, Doctor Beckett finds himself leaping from life to life, putting things right, that once went wrong and hoping each time, that his next leap will be the leap home.
Sam: Leaping about in time, I've found that there are some things in life that I can't change, and there are some things that I can. To save a life, to change a heart, to make the right choice. I guess that's what life's about}: making the right choice at the right time.
Al: If we knew the unknown, the unknown wouldn't be unknown.
Al: Oh, well, almost all animals can see me. But you know, there must be something weird lookin' about me, because I seem to intimidate them.
Sam: Maybe it's your clothes.
Al: [singing, trying to remember the lyrics] All around the something... the monkey chased the weasel... the monkey did something... and something happened to the weasel.
[about Al]
Sam: The fact that you were a practicing pervert at the age of 5 has nothing to do with the rest of the world!
Sam: Say something to me in Spanish.
Al: Uh, tu casa o mi casa.
Sam: My place or yours - Al!
[repeated line - season 1]
Sam: It all started when a time travel experiment I was conducting went... a little ka-ka. In the blink of a cosmic clock I went from quantum physicist to air force test pilot. Which could have been fun... if I knew how to fly. Fortunately, I had help. An observer from the project named Al. Unfortunately, Al's a hologram, so all he can lend is moral support. Anyway, here I am. Bouncing around in time, putting things right which once went wrong. A sort of time traveling Lone Ranger, with Al as my Tonto. And I don't even need a mask. Oh boy.
the Devil (as Al): What gives you the right to leap about time, putting right what I made wrong.
Sam: I'm just trying to get home.
the Devil (as Al): Well, you're not going to make it!
Al: Their only desire is for you to pamper them, and play with their...
Sam: Al!
Al: With their hair! Their hair!
Sam: What is she doing in Syracuse?
Al: I bet a lot people ask themselves that question.
Al: Don't tell me, let me guess, you've been invited to a costume party and you're going as a baked potato.
[his life in the 70's]
Sam: Great. I'm on the take, I wear polyester clothes and I live above a bar in an apartment decorated like a gym.
Sam: I can't have a life. All I do is live someone else's life. I right their wrongs, I fight their fights - geez, I feel like I'm Don Quixote.
Al: There are five stages of love. The first is denial, then the second is sex, then there's acceptance, then there's divorce... and then there's more sex, if you're lucky.
Sam: Who wrote this textbook, you or Zsa Zsa?
Al: Don't do anything I wouldn't do. And if you do, take pictures.
Al: I went over to check out the cheerleaders. Oh, Sam. There was one little girl who had these pommelos, man.
Sam: Pommelos are grapefruit.
Al: Pommel - that's my point!
[about Al]
Sam: At last, something sexual he's NOT into.
[repeated line - seasons 3-5]
Narrator: Theorizing that one could time travel within his own lifetime, Dr. Sam Beckett stepped into the Quantum Leap accelerator and vanished. He awoke to find himself trapped in the past, facing mirror images that were not his own and driven by an unknown force to change history for the better. His only guide on this journey is Al, an observer from his own time, who appears in the form of a hologram that only Sam can see and hear. And so, Dr. Beckett finds himself 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.
Al: Well, we been having some difficulty. Ziggy, he's, uh, going through mood swings. I think we need get a girl computer put it right next to him, one with a nice set of *hard* disks.
Sam: You would.
Sam: Where the hell were you?
Al: I was at the Laker game. It went into overtime.
Sam: A ball game? I nearly died because you were at a ball game?
Al: It wasn't just a ball game. It was a play-off game. At the party later, I met this dish named Martha.
Sam: I guess I can thank God you didn't spend the night with this Martha.
Al: Well, I did.
Sam: Do you have to sneak up on me?
Al: I'm sorry. What do you expect a hologram to do? Knock?
Al: We're making preparations for Tina's birthday party and she wants me to pop out of the cake. You'll never believe what she wants me not to wear.
[talking about God]
Sam: In case you haven't noticed, Al, the Committee isn't running this Project anymore... He is.
Sam: I'm running track, Al.
Al: Oh, well look, you pump your arms and you pump your legs and drive through the tape.
Sam: You were a runner too?
Al: No, but it sounds good, doesn't it?
Al: Women - you can't trust 'em. They don't understand the double standard.