When Back to the Future came out 25 years ago, I was too young to see it in theaters. But once I reached video-rental age I kept it—along with E.T. and Clue —in heavy rotation. Starring Michael J. Fox as Marty McFly, a 1980s teenager who accidentally travels back to 1955 and must make his way, as the title has it, back to the future, it's a natural kid's choice. Then again, it deals with topics usually left to adults or, more specifically, to Scandinavian adults: having sex with your parents and encouraging your parents to have sex in front of you.

In elementary school, I was only dimly aware of these incestuous themes. If you've graduated from eighth grade, you can't miss them. I rewatched Back to the Future on the occasion of its quarter-century anniversary (it's newly available in Blu-ray), and can attest that incest is absolutely central to the plot. What's genius about Back to the Future, however, is that it still manages to be unfailingly PG. It's a film about having sex with your family that's family-friendly.

To refresh: We first meet Marty, a skateboard-riding, guitar-playing teen, in 1985. His dad, George (Crispin Glover), lets himself get pushed around by his boss, Biff Tannen (Thomas F. Wilson). His mom, Lorraine (Lea Thompson), drinks too much. Depressed by these losers, Marty looks up to Doc Brown (Christopher Lloyd), who's just fashioned a time machine out of a DeLorean DMC-12. Then, following a Libyan terrorist shootout in a mall parking lot, Marty takes off in the DeLorean and ends up in 1955—the year Doc had the Eureka moment that eventually led to the time machine, and the year that Marty's parents fell in love.

Shortly after arriving in the past, Marty encounters his dad peeping at Lorraine from a tree branch. When George tumbles into the street, Marty saves him from an oncoming car and gets knocked unconscious. Big mistake. The driver is Lorraine's father, and the accident is what's supposed to bring George and Lorraine together. In this alternate history, it brings Marty and Lorraine together, instead. Lorraine develops a crush on her son, and now Marty must figure out not only how to get back to 1985 but how to transfer his mother's affections from himself over to his father. Otherwise, he and his siblings will never be born.

This seems icky. So icky that, when writers Bob Gale and Robert Zemeckis pitched the idea for Back to the Future to Disney, the studio purportedly said that a mother falling in love with her son was not appropriate for their brand. Universal green-lighted the project, but Disney's objection was not lost on producer Steven Spielberg, who once told Empire magazine that he "thought the Oedipal aspect was really gross."

Lorraine's infatuation never feels un-Disney, however. Back to the Future presents mother-son attraction not as a perversion, but as one of those things that just happens and kinda makes sense, if you think about it, since in this reality mother and son are the same age. Gale came up with the Back to the Future concept while flipping through his father's high-school yearbook. He thought, "Gee, if I went to high school with my dad, would I have been friends with him?" The Lorraine-Marty situation is not so different. If father and son had been contemporaries, would they have been friends? If mother and son had been contemporaries, would they have dated?

Lorraine, of course, has no idea that Marty is her son—she doesn't want her offspring, really, but a hot, young stranger. Marty realizes that Lorraine's his mother, so there's more room for viewer-horror when he remarks on her attractiveness. ("You're my ma, you're my ma … but you're ho … you're so, uh … thin" he stumbles.) What keeps this moment comic rather than Greek-tragic is that Lorraine is … thin; Marty's merely expressing an uncomfortable fact.

The Oedipal plot also feels harmless because we only get half of the Oedipus story—just the mother-son stuff, not the son-murdering-the-father part. Oedipus kills his father, Laius, at a crossroads. Marty saves George on a suburban street—a reversal, whether explicit or not, of that moment. Furthermore, Marty doesn't want to replace his father; on the contrary, he spends much of the movie trying to unite his parents.

Which brings us to the other weird thing about Back to the Future. Most people avoid thinking about their parents' sexuality. We accept that our parents have copulated at least once (or twice, or however many siblings we have) in the distant past and then drop all further inquiries. Marty can't drop it. He must involve himself directly with his parents' romantic lives in order to ensure his own existence.

Nowhere is this involvement, and its attendant strangeness, more blatant than at the Enchantment Under the Sea school dance, where George and Lorraine are supposed to share their first kiss. Up on stage as a replacement guitarist, strumming away to "Earth Angel" with Marvin Berry and the Starlighters, Marty watches his parents dance, and hopes that the kiss will actually take place—a guarantee that history is back on track. Stuck in the guitar strings there's a photograph that once depicted Marty and his elder siblings and that now depicts only Marty. The conceit is that if George doesn't kiss Lorraine, Marty's image will disappear, too, signifying that he'll never be born. If he does, Marty's image will remain intact, and his siblings will reappear. It's a clever visualization of an abstract drama, which also reinforces that Marty isn't merely waiting for his parents to lock lips but waiting for them to have sex and conceive his older brother, his older sister, and himself. He's eagerly anticipating a primal scene.

Watching your parents have sex (symbolically, at least) not once but thrice should probably result in severe trauma. Sigmund Freud thought that one of his more famous patients, Sergei "Wolf Man" Pankejeff, had witnessed such an episode (a tergo, no less), and blamed the event for his future neuroses. Of course psychic distress isn't especially appropriate for young moviegoers, so Marty feels none. Nope—after George kisses Lorraine, Marty's so ecstatic that he leads Marvin Berry et al. in a rousing rendition of Johnny B. Goode, which culminates in a rather bawdy, suggestive guitar solo. His parents' (symbolic) orgasm leads to his own. Otherwise put: Marty (symbolically) masturbates to his parents (symbolically) having sex.

Back to the Future was hardly the first science-fiction narrative to use time travel as a way to sidle up to incest. In Robert Heinlein's Time Enough for Love (1973), the protagonist "jumps" back to 1916 and falls for his mother. Unlike Marty, he actually consummates the relationship. What's unusual about Back to the Future is that it takes the stuff of Greek tragedy, and psychoanalytic case studies, and crams it into a PG format—by telling us there's really nothing to get upset about. Parent-sex isn't disgusting; it's joyous. Without it, we wouldn't be here.

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