I'm sure that in 1985, plutonium is available in every corner drugstore, but in 1955, it's a little hard to come by.
9 seconds sound clip from the Back to the Future movie soundboard.
You can hear this line at 00:53:29 in the Blu-ray version of the movie.
Quote context
[...]
- What the hell is a gigawatt?
- How could I have been so careless? 1.21 gigawatts!
- Tom, how am I going to generate that kind of power? Lt can't be done, can it?
- Doc, look. All we need is a little plutonium.
- Marty, I'm sorry, but I'm afraid you're stuck here.
- Whoa, whoa. Doc, stuck here? I can't-I can't be stuck here. I got a life in 1985.
- I got a girl. Is she pretty?
- Doc, she's beautiful. She's crazy about me.
- Look at this. Look what she wrote here, Doc. I mean, that says it all.
[...]