Plutonium
We watch in the opening scene as Doc's television set comes on and we learn that plutonium has been stolen. This is a symbol that there is great danger lurking ahead as plutonium is synonymous with nuclear weapons. And this is proved true when terrorists arrive and gun down Doc.
Fall
Marty tells the 1955 version of Doc that he fell and hit his head on the toilet and has come up with the flux capacitor--the key piece for time travel. This is a symbol that Marty is from the future and is what causes Doc to trust him.
Disappearing
We watch as Marty's hand begins to disappear while playing guitar during the dance. This is a symbol that Marty's reality is being changed as his father is allowing Lorraine to be taken away on the dance floor. If George doesn't act, Marty will be erased.
Time
We see clocks all over Doc's lab in the opening of the film. They are a symbol of time being his great experiment as he is working on time travel.
Johnny B. Goode
Marty has been playing guitar with the band during the dance and is asked to play another song. He plays Johnny B. Goode and grows it into a wild guitar solo. His solo is a symbol that these people aren't quite ready for what's to come in the future with this song.
