Doc: "How could I have been so careless? 1.21 gigawatts! Tom, how am I gonna generate that kind of power? It can't be done, can it?"
Marty: "Doc, look. All we need is a little plutonium."
Doc: "Oh" I'm sure that in 1985 plutonium is available in every corner drugstore, but in 1955, it's a little hard to come by. Marty, I'm sorry, but I'm afraid you're stuck here."