Movie
Back to the Future
Narrative tropes
Love Conquers All
highGeorge and Lorraine's romantic love is the literal plot mechanism: Marty ceases to exist (siblings fading from the photo) because their romance was disrupted, and their first kiss at the Enchantment Under the Sea dance directly restores his existence. Love motivates the decisive heroic act (George punches Biff to rescue Lorraine), love transcends the timeline paradox, and a character (Marty) is saved from non-existence by his parents' love. The entire mission is to ensure a specific act of love occurs.
About this trope: Love — romantic, familial, or platonic — is presented as the ultimate force that overcomes any obstacle including death, physics, evil, or cosmic forces. Love is a literal power.
Cultural messages
Family Is Everything
highProtecting family is Marty's sole motivation and the mechanism of resolution. The central threat is erasure of his family's existence. Marty sacrifices his own comfort and safety entirely for his family's sake. The emotional payoff is the transformed family at the end — George confident, Lorraine cheerful, siblings thriving — functioning as the story's climactic reward. Marty's desperate desire to return to 1985 echoes a 'there's no place like home' resolution, and the closing image of Doc arriving to deal with 'a problem involving their children' extends the family-protection theme forward.
About this message: Family bonds — biological or found — are ultimately what saves the day, provides meaning, and matters most. Characters who stray from family suffer; those who return are rewarded.
Movies that share these tropes
Full plot (spoilers)
In 1985 Hill Valley, California, teenager Marty McFly lives with his down-on-his-luck family: his father George is a timid, bullied pushover, his mother Lorraine is a depressed drinker, and his siblings are underachievers. After his band fails an audition, Marty meets up with his eccentric scientist friend Doc Brown in a shopping mall parking lot, where Doc unveils a time machine he has built into a DeLorean DMC-12, powered by plutonium stolen from Libyan terrorists. When the terrorists arrive to reclaim their plutonium, they shoot Doc. Marty flees in the DeLorean, accidentally triggering the time circuits at 88 mph and traveling back to November 5, 1955 — the day Doc first conceived the flux capacitor that makes time travel possible. Stranded in 1955 with no plutonium to return home, Marty inadvertently encounters his teenage parents. While spying on young Lorraine, George falls off a tree and into the path of an oncoming car; Marty pushes him out of the way, taking the blow himself. This disrupts the moment that was supposed to spark his parents' romance — now Lorraine becomes infatuated with Marty instead of George, threatening the family's very existence as Marty's siblings begin to fade from a photograph he carries. Marty tracks down the younger Doc Brown and explains the situation. Doc determines that the only energy source powerful enough to send the DeLorean back is a bolt of lightning — and a storm is forecast to strike the Hill Valley courthouse clock tower in one week. Marty devises a plan to get George to rescue Lorraine at their school's 'Enchantment Under the Sea' dance, recreating the moment that will make them fall in love. When the plan is nearly derailed by bully Biff Tannen, George finds the courage to stand up to Biff and knock him out. He and Lorraine share their first kiss, securing Marty's existence. Meanwhile, Doc strings a cable from the clock tower to channel the lightning strike into the DeLorean. After some near-misses with the cable connections, the bolt hits at the precise moment Marty reaches 88 mph, sending him back to 1985. Back in the present, Marty arrives moments before Doc is shot — but Doc survives, having taped together Marty's earlier warning letter and worn a bulletproof vest. Marty finds his family transformed: George is now a confident, successful science fiction author; Lorraine is cheerful and fit; his siblings are thriving; and Biff meekly works as George's detailer. Doc then reappears in the DeLorean, urgently telling Marty and Jennifer they must come with him to the future to deal with a problem involving their children.
Sources: Wikipedia, TMDb overview