July marks the 35th anniversary of Back to the Future, the enduring sci-fi and comedy classic starring Michael J. Fox as Marty McFly, an amiable teen who strikes up an unlikely friendship with Emmett "Doc" Brown (Christopher Lloyd). Thanks to Doc's DeLorean time machine, Marty winds up in 1955 to save Doc’s life and to make sure his parents (Crispin Glover and Lea Thompson) fall in love, thereby ensuring his existence.

Fans of the film have spent the past several decades wrapping their minds around the movie’s time travel paradoxes and missing pieces of the plot. Take a look at some of the most popular theories, then check out Back to the Future and its sequels on Netflix to see if they carry any weight.

1. Marty McFly’s parents knew he was a time traveler.

Perhaps the biggest mystery of Back to the Future is why George and Lorraine McFly fail to notice that their grown son Marty bears a striking resemblance to the man they knew as “Calvin Klein” who dropped into their lives in 1955 to make sure their romance was intact. One theory explained by Redditor djbred18 offers that George and Lorraine did recognize him. “I mean they had 30 years to figure it out!” the user said. Crucially, George heard “Calvin” using the names of Darth Vader and the Vulcan race from Star Trek years before they materialized, a fact any science-fiction author like George would have picked up on. A scene late in the film where Marty’s parents give him a brand-new truck and offer a knowing smile could be read as a thank you for his efforts.

Speaking with The Hollywood Reporter in 2020, Back to the Future co-screenwriter Bob Gale explained that they didn't make the connection: It was a simple case of Marty’s parents not recognizing the man they had spent just a few days with 30 years prior. “I would ask anyone to think back to their own high school days and ask themselves how well they remember a kid who might have been at their school for even a semester,” he said. “Or someone you went out with just one time. If you had no photo reference, after 25 years, you’d probably just have a hazy recollection.”

2. Doc Brown was suicidal.

While testing his DeLorean in the Twin Pines Mall parking lot, Doc Brown steps directly in front of the car traveling at 88 mph. The only way he wouldn’t be crushed is if his experiment succeeded and the car vanishes. Yet Doc makes mention of his other experiments being disappointing. Given his lack of confidence in his own abilities, standing in front of the car appears to be a death wish.

When asked about this theory by the Canadian Broadcasting Corporation (CBC) in 2018, Christopher Lloyd wasn’t buying into it. “I don’t think so,” Lloyd said. “Because Doc had so much confidence in what he was doing, he didn’t worry about that ... maybe a little doubtful, but Doc didn’t have a grim nature.”

However, Lloyd did add that: “You’ve given me a lot to think about though.”

3. Marty McFly’s actions altered his girlfriend’s appearance.

Elisabeth Shue, Michael J. Fox, and Christopher Lloyd in Back to the Future Part II (1989).Universal Pictures Home Entertainment

In the first Back to the Future, actress Claudia Wells portrays Jennifer Parker, Marty’s girlfriend. In 1989’s Back to the Future Part II, Elizabeth Shue took over the role because Wells was dealing with an illness in her family. For a series about time travel, it might be easy to explain why Jennifer’s appearance changes. According to Reddit user j1ggy, Marty’s presence resulted in unseen but demonstrative effects in the lives of Jennifer’s parents, possibly even resulting in Jennifer having a different mother or father. Because Marty seems slightly confused by Jennifer at the beginning of Back to the Future Part II, it’s possible he realizes he changed the past to the point that his girlfriend is now physically different.

4. Marty may have actually turned Biff Tannen’s life around.

At the beginning of Back to the Future, we see town bully Biff Tannen pushing around George McFly and demanding he perform Biff's work duties at their office. At the end of the film, Biff is in a subservient role, waxing George’s car as part of his work owning an auto detailing company. But, as Reddit user SatNav points out, that may have been best for Biff. He went from being dependent on George to assist him with his job to owning his own small business.

5. Doc Brown kills Marty.

At the conclusion of Back to the Future, time-traveling Marty returns from 1955 to witness 1985 Marty disappearing in the DeLorean. While that’s presumably Marty heading back to 1955, one theory has posited that Doc Brown is sending 1985 Marty either to his death or exiling him in time to make room for the returning 1955 Marty. Had he allowed 1985 Marty to continue living, he could have gone back to 1955 to meet the Marty already there. That, or two versions of Marty would have been running around Hill Valley in 1985.

Christopher Lloyd has dismissed this theory. “Doc would never send Marty off to his death, in any kind of scenario,” he told the CBC in 2018. “Doc couldn’t live with that.”