The third film in the Back to the Future trilogy contains a lot of backstory for the game, so even though it doesn't include as many of the rules of the Back to the Universe as its immediate predecessor, it's still important to understand how this film works as well.
Like the second film, Back to the Future Part III begins immediately where the last film left off. Doc had sealed the DeLorean up in a cave, written Marty a note informing him that he is working as a blacksmith in 1885, and left it with the post office to be delivered 70 years into the future.
Marty enlists the help of the Doc from 1955, as the DeLorean from the cave is in bad condition and needs to be fixed with 1955 parts. However, Doc notes that the hover equipment is damaged beyond repair and the DeLorean will probably never fly again. Upon reaching the location, they find Doc's tombstone in the cemetery and are surprised to learn that the date of his death is only one week after he sent Marty the letter. They go to the library and find a history book that proves that has a picture of Doc, taken in front of the clock of the clock tower, before it was finished being built, in 1885. They take a picture of the tombstone, which states that Doc was "shot in the back by Buford Tannen over a matter of 80 dollars". It also states that the tombstone was placed by Doc's "beloved Clara". Both of them have no idea who Clara is, but ultimately decide to proceed with the plan set up by the Doc from 1985, with one alteration.
The younger Doc and Marty decide to go against the wishes of the older Doc, who wanted Marty to go directly back to 1985 and leave him in the Old West. Marty instead intends to use the DeLorean to travel back to 1885 to bring Doc back home. Doc gives Marty his second silly disguise, and he takes the DeLorean time machine back to the old west. Of course, things don't work out as planned.
Upon arriving in 1885, the DeLorean winds up in the front of a group of Native Americans who are being chased by the United States Cavalry. Marty ends up driving the DeLorean into a cave before the Native Americans or the Cavalry can notice where he ended up. He finds that the gas line is leaking due to a strike by an arrow, but before he can attempt to fix it, he's chased out of the cave by a bear. Marty hits his head and is found by his great, great grandparents.
Upon entering town Marty takes the name of "Clint Eastwood", and then proceeds to embarrass Buford "Mad Dog" Tannen, who chases Marty with his gang, catches him, and strings him up on a noose to hang him. Marty is rescued by Doc, who shoots the rope from which Marty is hanging with a homemade sniper rifle. Buford informs Doc that he owes him eighty dollars for a horse that he shot after the horse lost a shoe that Doc had put on the horse. Doc refuses to pay as Buford never paid to have the shoes put on the horse in the first place. Marty then informs Doc that he will be killed by Buford in a week due to the fact that Doc didn't pay him the money.
Doc also learns of the damage to the time machine, but Marty seems unfazed as he is under the mistaken impression that the Mr. Fusion nuclear reactor that Doc used to convert garbage to energy can be used to power the DeLorean. Doc corrects Marty, as he explains that the Mr. Fusion is only used to power the fusion generator of the time circuits in place of the plutonium that it previously used. The DeLorean itself still uses a standard combustion engine to run and needs gasoline in order to function.
Several test runs of ideas ranging from using alcohol to fuel the DeLorean, to pulling the DeLorean with horses, fail. Doc then decides to instead use a steam locomotive to push the DeLorean up to eighty-eight miles per hour. They learn that a train will be coming by the morning that Doc is supposed to die and decide that, although it is cutting it close, they should be able to make it if they head right to the train and avoid Buford.
However, Doc and Marty soon find a woman whose horses have been spooked by a snake and is about to head into the ravine. Doc saves her and discovers that she is Clara Clayton, a school teacher. They fall in love at first sight, which Doc had originally passed off as an impossibility for his scientifically oriented mind. Marty tells Doc that the ravine is known in 1985 Clayton ravine. It was renamed from Shonash ravine after a teacher fell into it and died one hundred years earlier. Doc realizes that Clara was meant to die in that ravine, and is concerned that he had changed history.
In the previous timeline, before Marty went back to get Doc, Doc had volunteered to pick Clara up at the station. That gave them a week to get to know each other and led to Clara making the epitaph on Doc's grave when he died. In this timeline, however, Marty's appearance caused Doc to forget to pick her up, which led to the snakes spooking the horses as they did before Doc traveled to 1885. However, Doc rescued her, leading them to fall in love as they had in the timeline before Marty arrived.
Clara tells Doc that she hopes that she'll see him at the Hill Valley Festival. Doc states that she will, and both he, and Marty, as "Clint Eastwood", attend the festival. While there, Mad Dog Tannen smuggles in a gun, which has only one small bullet that is said to take a week for the injuries to kill a man. Doc realizes that this is where he died, so he purposely keeps facing Tannen. As Mad Dog is about to shoot Doc, Marty uses a Frisbee brand pie tin to knock the gun out of his hand, causing the bullet to merely graze Doc's hat.
Mad Dog calls Marty "yellow", which gets at his weakness, which is to never allow anyone to call him chicken. He agrees to a duel on the day of the train's arrival, assuming that they will be out of there by then. However, Doc points out that it is possible that the train might be late. Marty's great-grandfather is disappointed in "Clint", and tells him the story of his brother Martin who was killed by a stabbing because he would always do foolish things simply because someone called him a coward.
Later, Marty points out that the picture of the tombstone no longer states that Doc died, so he assumes that their plan will work. However, Doc points out that if the future was already written, it doesn't make sense for the tombstone and the date of the death, as well as for the picture itself, to still exist. Doc tells Marty that the future still is being written, and it is still possible that Marty will be the one to be killed on that day.
Doc and Clara fall even more in love when it is revealed that they both share a love of science, with him being a scientist and student of all sciences, and her loving science since her father gave her a telescope as a child. They both also love Jules Verne novels, as his words were what inspired both of them to be interested in the pursuit of scientific knowledge. Doc decides that he wants to take Clara with them, so he tries to tell her the truth. She thinks that he is abusing her love of Jules Verne to make lies to break up with him, and tells Emmett that she never wants to see him again.
Doc gets depressed from his heartbreak and goes to the saloon to pour his heart out. The patrons at the bar find his stories of the future humorous, and when Marty arrives, on the day of the duel, he thinks that Doc is drunk. Marty convinces Doc that they need to go, and Doc drinks the whiskey in front of him. He then passes out. The bartender tells Marty he only had one drink and is known for not being able to hold his liquor.
The bartender gives Doc a drink to make him sober, and everyone waits for Doc to wake up. Marty then hears Mad Dog Tannen outside and discovers that it is now "Clint Eastwood" who is stated on the photograph to die. He tries to sneak out the back, but Doc, who had awoken from his drunken sleep, is captured by Mad Dog. Doc tries to get Marty to leave and save himself, but Marty can't leave his friend behind.
Marty goes out to face Mad Dog but drops his gun telling him that he won't fight. Mad Dog then proceeds to shoot him and cockily walks up to what he thinks is "Clint's" dead body. However, Marty had taken a door from a stove, and used it as a make shift bullet proof vest, as he had seen in a Clint Eastwood film shown at Biff's Pleasure Paradise in Back to the Future Part II. Marty then knocks Mad Dog Tannen out, and the lawmen proceed to arrest Tannen and his gang. At this point, the picture now just shows grass, which means that the future hasn't yet been completely written (possibly indicating that Doc plants the tombstone at some point in order to make sure that Marty arrives in the past to prevent a paradox).
Wearing bandanas over their mouths like outlaws, Marty and Doc hold up the train and claim that it is a science experiment. They disconnect the engine from the other cars and drive it up to the DeLorean, which is already on the unused portion of the tracks. Doc has prepared his own type of presto logs, which are color coded to get hotter and make the train go faster once it reaches certain speeds. Marty is in the DeLorean from the start, and Doc plans on catching up once the logs reach a certain level.
However, Clara, who learned that Doc was torn up about their breakup, and discovered Doc's model DeLorean time machine in his workshop, had jumped from her horse to the moving train. Doc is just moving to the DeLorean when he hears the whistle blow. He sees Clara and goes back to rescue her. However, at this point, the DeLorean has passed the point of no return, and it's too late. Marty slides the hoverboard back to Doc, and he safely glides away with Clara in his arms. Marty then successfully goes back to 1985, as the train in 1885 crashes in the ravine.
The DeLorean crosses the now completed bridge across the ravine, which is now named "Eastwood Ravine". Marty hears a train whistle and jumps out of the DeLorean just before the collision. However, the DeLorean is broken into pieces by the train. Marty states that Doc has gotten his wish, as the DeLorean has been destroyed.
Marty then goes and gets Jennifer, who at first thinks that everything was a crazy dream. However, once they reach Marty's classmate Douglas Needles, he tries to coax Marty into drag racing him by calling him chicken. Marty then pretends to comply, but backs up, as, after the advice given by his great, great grandfather, he has learned to not be intimidated by bullies any longer. Marty sees Needles narrowly miss a Rolls-Royce and realizes that he would have hit it. Jennifer then realizes that it wasn't a dream and pulls out the fax she took in the future. She sees the message that Marty has been fired become faded until it is erased.
Marty and Jennifer go to the train track to look at the wreckage of the DeLorean, and they become sad that they will never see Doc again. At that moment, Doc appears in a steam-powered time train. He and Clara now have young sons, Jules and Verne. Doc had previously come back to the 1980s to pick up Einstein and had traveled to the 2010s to have the train hover converted. Doc and his family then leave to the past, and the original trilogy comes to a close.
However, the story is continued in Back to the Future: The Game, which takes place six months after Doc leaves Jennifer and Marty in 1985 in his time train. The story is certainly a fun one, but it takes its cues from the second film in terms of confusing moments.
But first, we'll take a detour towards IDW's comics, as they explain a lot of mysteries from the films.