The following is from an article titled "The Other Marty McFly?" by Bruce Gordon published in Starlog #108 (July 1986):

"Our mysterious voyager, the tale begins, appears in Back to the Future for just a fleeting second, on screen for only a handful of frames. Cast your thoughts back to the film … back to the parking lot at Twin Pines Mall. The terrorists have arrived, and they're aiming their machine guns right at Doc's heart. Their van is parked at the screen's left-hand side, and Doc is standing to the right, holding a pistol in his hand. He raises his arms into the air, and then, just behind Doc - between him and the truck - we can see a spot of light from one of the nearby stores. Watch that light.

At the very instant that Doc tosses his pistol to the ground (and all eyes in the audience are following its path across the pavement), a silhouetted figure steps into that light. Less than a second later, the figure is gone, Doc has been shot and the chase is on.

Who was that figure? Where did he come from? Why was he there?

The answer to the first question is easy: The figure was Marty McFly, of course. Where did he come from? He had just returned from 1955. And why was he there? Let's find out!

… Send your thoughts into the past again, this time towards the film's end. Marty runs to the mall, and arrives just in time to see Doc shot again, and to see another Marty hop into the DeLorean and drive off through time. So, there are two Martys during that climax - one arriving from Town Square, and one departing for 1955.

Logically, then, if there were two Martys in the mall at the film's end, then there should have been two Martys in the mall at the beginning . . . one arriving, one departing. The silhouetted figure in the light must be the 'arriving' Marty.
Let's see if we're right. Taking a look at the accompanying chart, we find two pathways. The pathways represent two parallel worlds - each existing simultaneously in different dimensions of time. Each step in a path is a specific event that's shown, told, or implied somewhere in the film. The white steps (Note - unable to reproduce the chart) represent the actions of our Marty McFly, and they trace the exact story of the film. The grey steps (Note - unable to reproduce the chart) follow the journey of the other Marty, the mysterious figure we saw - or probably didn't see - silhouetted by the light. (From here on, let's call this mysterious figure 'Marty II'.)

Naturally enough, we begin at 'Start,' which is where the movie opens. We'll call this path 'Our Dimension.' The first item we find reads, 'The Ledge is Not Broken.' In the scene where Marty and his girl friend Jennifer are sitting on the bench in town square, planning their big weekend, the concrete ledge on the clock tower is intact. Doc's foot hasn't broken it off because Doc hasn't climbed up to fix his cable. Not yet - and in this dimension, not ever.

Following down the chart, we see the terrorists arrive at the mall, followed a moment later by the mysterious Marty II, silhouetted in the light. Then, Doc is shot and our Marty escapes back to 1955. Notice that our Marty not only goes back in time, but it looks as if he switches over into the opposite dimension! Actually, what happens I is this: Our Marty's leap to 1955 and his interference in history causes the split that creates these parallel dimensions!

Our Marty arrives at Twin Pines Ranch, crashes into the barn, and while escaping in a hail of shotgun fire, runs down one of Peabody's twin pine trees (more about that later). Marty prevents his parents from meeting, but manages to get them back together just before his rendezvous with Doc at the clock tower.

Marty also gives Doc the all-important note that warns him about the eventual terrorist attack. Doc finds the note and starts tearing it up when a tree branch falls and pulls apart Doc's cable. Doc sticks the note in his pocket (you weren't distracted by all the other action, were you?) and climbs up the tower to fix the cable. He slips and breaks the concrete with his foot.
Now, here's where the plot gets tricky. This is where we leave the movie's story for a moment, and take a look at the 30 years between 1955 and 1985. Marty and the DeLorean make contact with the lightning and leap over those decades in an instant, but everyone else - including Doc, George, and Lorraine - have to live them out one day at a time.

During those 30 years, George and Lorraine become a popular and successful couple, thanks to our Mary's involvement. In a few years, they get married, and before long, they have a son. And who is their son? Why, he's Marty II, of course - the other Marty! After all, our Marty is already 17 years old and leaping through time in the DeLorean.

Over the years, Marty II grows up. He and Doc become friends. But remember - Doc spent a week with our Marty 30 years ago, so Doc already knows everything that's going to happen. He knows about the DeLorean, time travel and Marty's escape back into time. And somewhere along the way, he tapes Marty's note back together, and finds out about the terrorists.

So that brings us back to October 26, 1985 as our Marty arrives in Town Square and crashed into the theater. He has returned to 1985, all right, but he's still in the opposite dimension! As the helicopter hovers overhead, we see that the ledge on the clock tower is broken. When our Marty sees the terrorists drive by in their van, remember that we're in the other dimension, and Marty II is already down at the mall with Doc, testing the DeLorean. Our Marty runs to the mall and grabs onto the big sign . . . which now reads 'Lone Pine Mall.' (Remember how Marty ran down that pine tree in 1955? That was the end of the Twin Pines breeding experiment!)

Our Marty watches in horror as Doc is shot and Marty II drives off in the DeLorean. But Doc is OK - he has taped Marty's note back together, learned about the terrorists, and put on a bullet-proof vest.

Doc and Marty rescue the stalled DeLorean from Town Square, and Doc drives Marty home. But what does Marty find? He finds new parents, a new brother and sister, and a new 4X4 Toyota truck parked in the garage. How is this possible? This is Marty II's family! In this opposite dimension, Marty I has traded places and stepped into the life that was being led by Marty II. It's Marty II's father, mother, brother and sister - and that's Marty II's truck parked in the garage!

Where, then, has Marty II gone? He has time-warped out of the shopping mall and gone back to 1955! Let's chase after him now, starting at the top of the chart and following the grey steps. Notice that Marty II has also wound up in the opposite dimension - the dimension where Marty I used to be.

Marty II arrives at Peabody's Twin Pines Ranch, only this time he doesn't run down one of the pine trees. (Remember, the off-road Toyota 4X4 parked in the garage belonged to this Marty . . . maybe he drives better than our skateboarding Marty. So, the pine trees survive, Peabody's Twin Pines breeding experiment continues, and 30 years later, the big sign at the mall reads 'Twin Pines Mall' - just like it did at the movie's beginning.

Marty races out of the Peabody field and stops on the road in front of his home-to-be. He gets out of the DeLorean, reloads the plutonium, and returns to 1985.

'Now, wait just a minute,' everybody moans. 'Where did Marty II get the extra plutonium?' That's easy - Doc loaded it into the DeLorean. Remember that Doc had met our Marty 30 years ago, so he already knew Marty II was going to go back in time, and he knew that Marty II would get stranded without any plutonium for the return trip. Now, Doc may not be willing to alter history by trying to stop Marty II from going back in time, but he is willing to gamble on a little insurance. So, the first thing he does when he takes the DeLorean down to the mall is to load that spare plutonium.

'Well, OK,' someone says. 'But why does Marty II turn around right away and return to 1985?' Remember the speech that Doc gave our Marty when they first met at Doc's house back in 1955? 'You must not see anybody, you must not talk to anybody,' Brown warns. 'To do so would have serious repercussions on future events!' Our Marty had already broken that rule, so it was too late for the warning to do any good - but it wasn't too late to warn Marty II. So, it's logical that when Doc was showing the time machine to Marty II, he would explain how important it was not to interact with anyone in the past, and to get back to the present as quickly as possible. We know that Marty trusts and respects Doc, and so we can assume he follows the instructions to the letter, reloads the plutonium, and jumps back to 1985 - without meeting anyone.

We'll also assume that Marty II set the DeLorean controls to return minutes early in an effort to save Doc. Unfortunately, he's not going to be any more successful than our Marty was.

Now, it's Marty II who's jumping over the 30 years between 1955 and 1985, just as our Marty did. And just as before, George, Lorraine, and Doc live those 30 years one day at a time - only this time, it's without the 'benefit' of our Marty's involvement. Let's keep following the grey path and see just what happened during the 30 years that Marty II skips over. This part is easy, because the whole story is recited by Lorraine over dinner at the film's beginning. It goes like this: George gets hit by the car, he and Lorraine fall in love at the dance, get married and have a son named Marty. Our Marty. Marty I.
Over the years, our Marty grows up and becomes friends with Doc. Only in this dimension, Doc never met Marty back in 1955. Doc doesn't know about the DeLorean, or about time travel. He doesn't have a note to read, so he doesn't know to protect himself from the terrorists. When Doc goes to the mall on October 26, 1985, he's not wearing a bullet-proof vest, just his open shirt and his cotton underwear. And finally, back in Town Square, the clock tower ledge isn't broken, because no one was ever up there.

You'll notice by our chart that we're back to the start of Back to the Future but we're looking at it from a whole different perspective! When Marty II appears as a silhouette in the light, we know who he is this time, and where he's coming from.
And while our Marty came running up to the sign because he was coming from Town Square, where the Delorean was stalled in the street, Marty II didn't reappear in Town Square, because he never went there. He stayed on the highway near his home-to-be. So, when Marty II reappeared in 1985, he was coming from another direction. That's why he ran in front of the light, instead of over by the sign.

Our next step along the grey path says that Marty II runs to Doc and finds him killed. Doc never had the warning from Marty in 1955. He wasn't wearing the bullet-proof vest. Doc is dead.

From here on out, believe it or not, things get worse. Marty II heads home alone, and finds he has a brand new family. Just like our Marty, he's traded places - only now he's stepped into the life that was being led by our Marty. Now, his father is a failure, his mother is an alcoholic and his brother and sister are nerds. And the Toyotat 4X4 that he left parked in the garage the night before has turned into a skateboard, and even that got left in the parking lot back at the mall! (It's a sad ending. No wonder they filmed the other version!)

Now we've finally reached the end of our chart. But wait, our story's not over. Marty II still has the DeLorean and extra plutonium - maybe he can go back in time again, and try once more to save Doc's life. What a sequel that would make!

Bruce Gordon, an Imagineer at WED Enterprises, is involved in the future development of Disneyland. He co-authored 'Tomorrowland 1986' in Starlog #98-99. Assistance on this essay was provided by Chris Tietz and David Munford, who made substantial contributions to solving and explaining the mystery of Marty II.