Doc Brown

In 1985, Doc shows Marty the plutonium-powered time machine, plutonium he took from Libyan terrorists. They attack him, and Marty flees in the car. In 1955, the younger Doc learns that only lightning gives the 1.21 gigawatts needed to return. In 1985 he turns up alive: he taped Marty's letter back together and wore a bulletproof vest.

Doc arrives in 1985 in a hurry and takes Marty and Jennifer to 2015 to help their children. He leaves Jennifer asleep in an alley so she won't learn her future. He finds out Marty bought an almanac and warns him about profiting from time travel. In the alternate timeline, this Doc is in a mental hospital. Later, lightning throws the DeLorean to 1885, and he writes his friend a letter.

In 1885, Doc hides the car and writes to Marty. He saves Clara from the ravine and falls in love. Resolved to leave, he goes to say goodbye to Clara and tries to drown his sorrow at a bar, but passes out after a single drink. He then uses a locomotive to push the DeLorean to the right speed and the hoverboard to save Clara. At the end, he returns with his family in a locomotive that travels through time.

Christopher Lloyd took the part after a friend encouraged him. Per Wikipedia, the wild hair came from Albert Einstein and conductor Leopold Stokowski, and Lloyd adopted a hunched posture to come closer to Michael J. Fox's height. John Lithgow was suggested earlier but was unavailable.