The time loop is pretty much a classic science fiction trope, thanks in large part to the enormous success of the 1993 film Groundhog Day. It's been used so often, in fact, that it's challenging to come up with a fresh take. But the Netflix series Russian Doll and the new film Happy Death Day 2 U manage to do just that, giving us time loops with a multiverse twist.

Wikipedia has amassed an impressive list of films featuring time loops: 49 so far, and that's not counting TV shows, like The X-Files episode "Monday" (in turn referenced on a Buffy the Vampire Slayer episode, "Life Serial"). The earliest film dates back to 1933: Turn Back the Clock, in which a tobacconist named Joe is killed in a hit-and-run and wakes up 20 years earlier. But it's not a true time loop tale, having more in common with It's a Wonderful Life.

A 1987 Russian film, Zerkalo dlya geroya (Mirror for a Hero), does have a lot of the key elements in place. But the real original source material is probably Richard A. Lupoff's 1973 short story, "12:01 PM," adapted into an Oscar-nominated short film in 1990 and a full-length feature in 1993—the same year Groundhog Day came out. (Lupoff definitely noticed the similarities and considered suing for plagiarism, but eventually dropped the idea.) It's pretty much been a sci-fi mainstay ever since.

(Some spoilers for both Happy Death Day films and Russian Doll below.)

Happy Death Day 2 U is the sequel to last year's Happy Death Day, a comic-horror film with the emphasis firmly on the comedy—a cross between Scream and Groundhog Day. Sorority sister Theresa "Tree" Gelbman (Jessica Rothe) is murdered on her birthday by a killer in a Babyface mask and finds herself reliving that day over and over. (Babyface is the fictional Bayfield University's mascot, and they should really rethink that choice.) She takes advantage of the time loop to solve her own murder, and maybe get some closure over some personal trauma in her past. Bonus: she also snags a nice guy boyfriend, Carter (Israel Broussard). There's even an overt nod to Groundhog Day at one point, with Tree confessing that she's never seen the film.

"A day reset when you died? I might be able to help with that."

The film was overshadowed by the stellar Get Out—both were produced by Blumhouse Productions and released the same year— but it still grossed $122 million worldwide against its modest $4.8 million budget. Director Christopher B. Landon knew that if he made a sequel he'd want to focus on why Tree ended up in a time loop in the first place, rather than produce a straight retread. So Happy Death Day 2 U picks up right where the original left off.

Groundhog Day may have inspired the original, but Happy Death Day 2 U pays particular homage to Back to the Future II—a film Tree also hasn't seen. Carter's roommate, Ryan Phan (Phi Vu) walks in on Tree and Carter in his dorm room the day after her never-ending birthday, and says he's having an extreme case of déjà vu. "A day reset when you died?" Tree says. "I might be able to help with that. I died 11 times."

Babyface is back

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Blumhouse Productions

Ryan is a physics student, working on a mysterious project that might be responsible for the original time loop occurring in the first place. Alas, when he and fellow physics students Samar (Suraj Sharma) and Dre (Sarah Yarkin) attempt to close the loop, something goes terribly wrong, and a frustrated Tree finds herself back in the original time loop, once again reliving Monday the 18th. At least she knows who the killer is. Right?

Not so fast. Tree discovers she's been blasted into a loop in an alternate timeline, where things are just a bit different. For instance, while Babyface is still on the loose, the first film's killer is now one of the victims. Tree enlists the aid of the whole crew to figure out how to return to her own timeline. This requires a lengthy trial-and-error process. Rather than be repeatedly killed by Babyface to buy time, Tree opts for a series of creative suicides to reset the day in this new timeline. And she still has to catch the killer.

Much of the credit for making this work belongs to Jessica Rothe, a gifted comedic actor who attacks the role of Tree with gusto, making her likable even when she's losing her cool and screaming at everyone about her predicament. The supporting cast is great, too, especially Rachel Matthews as sorority sister Danielle. If there's a weakness, it lies with the rather messy plot—most notably the decision to have a physics experiment cause the time loop.

Granted, Einstein's general relativity does technically allow for the possibility of spacetime curving so dramatically that it folds back onto itself, creating a temporal fold where the present touches a point in the past. It would just require a huge amount of energy, more than all the energy in our galaxy combined. Technically it's called a closed timelike curve, often used in science fiction as a plot mechanism for enabling time travel to the past. The time loop is another type of closed timelike curve, where events unfold forward in time, yet always end up right back at the same starting point.

Someone caught in such a loop really wouldn't be able to change anything, nor would they remember past cycles the way Tree does (never mind the issue of why she's the only one who remembers). This would violate causality, Stephen Hawking once argued, citing a "chronology protection conjecture" whereby the laws of physics conspire to prohibit such things to prevent troublesome paradoxes. Happy Death Day 2 U doesn't bother with even a passing reference to any of this, opting instead for vague science-y terminology that doesn't really make much sense. But that's only likely to bug hard-core physics nerds like yours truly, and I still genuinely enjoyed the film.

A bug in the code

If Happy Death Day 2 U is a slyly silly sweet confection, then Russian Doll is its darker, salty, punk sitcom sibling, with eight tightly paced half-hour episodes—a welcome novelty in this age of slow-burn peak TV. Co-created by series star Natasha Lyonne with Amy Poehler and Leslye Headland, the plot centers on a chain-smoking game developer named Nadia Vulvokov (Lyonne), who dies repeatedly the night of her 36th birthday party and keeps looping back to the host's funky East Village bathroom. (The song playing with every reset is Harry Nilsson's cheerfully bleak "Gotta Get Up," which you may come to hate, but the entire soundtrack is aces; there's already an official Spotify playlist.)

Netflix

Netflix

Netflix

Netflix

Netflix

Netflix

Netflix

Netflix

Netflix

Netflix

Netflix

Naturally, Nadia wants to get to the bottom of what might be happening, although her sleuthing is hampered by repeated accidental deaths—including several falls down an especially treacherous staircase. Her first hypothesis is that it's a bad drug trip, but that seems unlikely since she's an avid recreational user who's never met a drug she didn't like. Then she meets a young man named Alan (Charlie Bennett) in an elevator, and realizes there might be a crucial connection between them.

Russian Doll is a weird, ribald, thoroughly enjoyable ride that deserves every bit of the praise it has received; the sight gags and whip-smart dialogue are priceless. And, again, much credit goes to Lyonne as its raspy-voiced star. Nadia describes herself as the offspring that Charles Bukowski and the young girl from Brave might spawn. (Alan, in a fit of pique, calls her a "carcinogenic siren.") She's smart, tough, uncompromising, foul-mouthed, and unapologetic about her lifestyle, but haunted (like Tree) by past trauma and regret—and also capable of shining moments of generosity and selflessness.

Both Happy Death Day 2 U and Russian Doll are tales of a woman caught in a deathly time loop on her birthday, who must figure out how to break the loop and achieve some self-realization in the process. While Tree thought she broke the original loop by making peace with her past, in the sequel she discovers that had nothing to do with it; Ryan's physics experiment both caused and closed it. She decides that doesn't make what she learned any less meaningful. Nadia refuses even to consider a moral component to her dilemma (although she is not a "nice" person by the usual definition), preferring to think of it as "bug in the code" in one of her video games. Find the bug, fix that, and the game will stop crashing.

"Life is like a box of timelines."

Tree must grapple with the challenge of an alternate timeline. Similarly, over the course of her own occasionally tawdry adventures, Nadia also starts to ponder the possibility of a multiverse, wondering what happens to everyone else when she dies on the night of her party. Perhaps, she thinks, the universe splits every time she dies. "Life is like a box of timelines," she tells Alan's elderly neighbor when he chides her for smoking, because his wife smoked and died of cancer. She points out that there could easily be another timeline where his wife is still alive and happily living with Fabio. ("Not Fabio. He's too good-looking and I'd be jealous," the neighbor replies.)

That's pretty much the reasoning behind the "Many Worlds" interpretation of quantum mechanics, proposed in the 1950s by a nuclear physicist named Hugh Everett III. Like many of his scientific colleagues, Everett was deeply dissatisfied with the troubling implications of quantum mechanics. Any quantum system is described by an equation known as the wave function, which contains the probabilities for every possible outcome. Until a measurement of some kind is made, all those possibilities exist in a superposition of states. "Observing" the system causes the wave function to collapse, reducing all those possibilities to a single "real" event.

But what happens to those other possibilities? The textbook interpretation of quantum theory has no good answer for this, and simply assumes all the other potential outcomes vanish by necessity once a measurement is made. Everett came up with his own ingenious solution: perhaps, he reasoned, a good wave function never dies—it just fails to collapse. It continues to evolve instead, forever splitting into other wave functions in a never-ending tree, with every branch becoming an entire universe.

Alternate timelines are everywhere these days in science fiction. It's a testament to Everett's vision that the notion of Many Worlds has become so familiar that we're even seeing it referenced in comic horror films and a Netflix sitcom.

Happy Death Day 2 U is currently playing in theaters. Russian Doll is streaming on Netflix.