Doc (Christopher Lloyd) is happily living as a blacksmith in 1885, where the fairy tale cowboys and indians roam. Marty (Michael J. Fox) receives Doc's letter saying not to come back for him as he is happy where he is but finds a tombstone which outlays Doc's date and cause of death and enlists the help of the 1955-Doc to go back and prevent Doc's death.
After arriving in 1885, Marty finds there is no fuel - a fueling station isn't due for another 50 years - to get the Delorian up to speed, so Doc and Marty must find a way to get the Delorian to 88mph using only rudimentary 19th Century technology in order to escape the past and return Back To The Future.
Much like Back To The Future: Part II, Part III expands on the ideas of the original film while following a very familiar formula. Much of the storyline is heavily borrowed from the first two films - the big adjustment is that the idea of having no fuel is being explored.
Part III is also the most isolated of the films, with less well known music - a factor due in the most part to the exit of the 1980's. The set design is vastly different, mostly due to the fact that Hill Valley is under huge development. The now famous clock tower is currently under construction which aids with much of the back story on both of the first two films.
This film allowed Christopher Lloyd to show another side to his acting with the introduction of a family for him. His eccentricity is not diluted as a result - much of the humour is based on this - but it does enable the development of the Doc character which Lloyd portrays well.
The ending is as outlandish as the rest of the film, further adding to the absurdity of the ending of the first film, but it does leave on a poignant note. Director Robert Zemeckis provided an ending that enabled further films to be added - but in the knowledge that they wouldn't live up to his original trilogy. To date, no-one, thankfully, has taken on the task. It wouldn't be the same.
As with the second film, the originality is lost following the first film, but still highly recommended.