Two men wake up in an abandoned industrial bathroom, each chained to a pipe. Once they find hacksaws in a toilet tank, they attempt to escape from their thick chains, to no avail.

That is when a doctor played by Cary Elwes has a realization about their captor that brings the impossible choice of “Saw” into visceral focus: “He doesn’t want us to cut through our chains. He wants us to cut through our feet.”

“Saw,” which was released 20 years ago this week, was never supposed to be a success. It was meant as a low-budget calling card for two Australian film students who were looking to start their careers with a scrappy splash and a neat idea: a serial killer who forces his victims to prove they want to stay alive through bloody sacrificial tasks.

Instead, it made more than 80 times its budget at the box office, created a horror villain for the new millennium and inspired the label “torture porn,” with each installment in the ensuing franchise ratcheting up the disgust.

It can be hard to remember two decades later that the original “Saw” was less interested in gore than morality.

“What would you do to save your life, or someone else’s?” said the actor Shawnee Smith, summing up the central tension behind the franchise’s elaborate traps. In order to expediently escape the “reverse bear trap” enclosing her head in “Saw,” Smith’s character is forced to locate a key inside another person’s stomach. He is drowsy but still alive.

Elwes, known for playing Westley in “The Princess Bride,” said that “Saw” had appealed to him because it was a horror story embedded with ethical and moral questions.

“It’s a thinking person’s thriller,” he said.

The germ of the idea that became “Saw” was a simple one: two men, locked in a room, with a body lying between them. James Wan and Leigh Whannell, film school classmates at the Royal Melbourne Institute of Technology, had been looking to develop their first feature and kicked around ideas until one stuck. They wrote the script together and Wan directed.

Wan, who has gone on to make big-budget Hollywood fare as well as unhinged horror flicks, like “Insidious” and “Malignant,” said “Saw” turned classic tropes on their heads.

“Oh, you’ve seen that? Oh, you think you know what’s going to happen?” Wan said from a dingy basement bathroom in London, a fitting homage to “Saw” and the only place he could get cellphone service while shooting the next installment of “The Conjuring” franchise there. “I’m going to try something else you haven’t seen before.”

The classic tropes within “Saw” — serial killers, haunted detectives, creepy puppets — are mashed together and inverted around the undeniable binary of its core choice.

Unlike the winking meta horror fare that preceded it, including horror buff “Scream” teens and Rube Goldberg machinations in “Final Destination,” no amount of genre knowledge can save the characters of “Saw” from having to make an ultimate decision.

The film critic David Edelstein described “Saw” and some of its peers — most notably Eli Roth’s “Hostel” (2005) — as “torture porn” in a 2006 New York magazine essay. But the first “Saw” is not especially gruesome when compared with the deaths punctuating this year’s “Terrifier 3” and “In a Violent Nature.”

“Saw” is more of a detective thriller with a gruesome twist. The titular tool acts as a rusty Chekhov’s gun, while the desperation of the characters provides the terror.

The franchise’s traps are devised by the character John Kramer (Tobin Bell), a terminally ill cancer patient who is angry at people for wasting their lives. Kramer, who would eventually be known as Jigsaw, is technically not a serial killer. Every victim is given the choice to live.

“Deep down inside, although he is of course somewhat ruthless in this approach, John’s in the business of saving souls,” Bell said.

That framework came out of Whannell’s personal experience with recurring migraines in his early twenties. While sitting in a waiting room before a CT scan, he wondered how the diagnosis of an aggressive brain tumor could change someone.

“The game seemed to be about time,” Whannell, who also plays the second man trapped in the bathroom, said of the killer’s motives. “He’s giving them this time limit and he’s making them do these extreme things, like, ‘How badly do you want to live?’”

“Saw” did not have a simple path to creation. When it came time to actually make the movie, Australian financing fell through.

Wan and Whannell were ready to scrape together money to film on the cheap, but Whannell’s manager, Stacey Testro, thought it could work in Hollywood, where she also had an office. Instead of sending her to court producers in the United States with just a script, the writers made a 10-minute short film of the reverse bear trap sequence with Whannell in the rig.

Testro remembers watching the short in her living room, with her luggage by the door and a car out front waiting to take her to the airport. “It was just thrilling,” she said.

Producers at Evolution Entertainment — Gregg Hoffman, Oren Koules and Mark Burg — were impressed enough to give Wan and Whannell a $700,000 shooting budget for the chaotic 18-day shoot, which cut costs by filming mostly in the bathroom set. Wan, who had previously focused on stop-motion animation, also handmade Billy the Puppet, an unsettling ventriloquist dummy with red spiral cheeks, a tricycle and the franchise’s most famous line, “I want to play a game.”

Wan used papier-mâché, table tennis balls for Billy’s eyes and cardboard boxes to fill out a child-size tuxedo for the body. (At first he also had a bowler hat, Wan said, but it looked slightly too silly.)

The limited budget did not prevent the movie’s ideas from resonating with audiences.

“The first movie was all about ethics and how far can you go and what’s right and what’s wrong with humanity,” Burg said, recalling that Evolution thought “Saw” would be its version of David Fincher’s “Se7en” (1995).

Whannell described his goal for “Saw” as a mix between “Se7en” and “Cube.” In Fincher’s movie, Brad Pitt and Morgan Freeman are detectives chasing down a serial killer who models his murders after the seven deadly sins, punishing his victims via elaborate setups symbolic of their moral failings. In “Cube” (1997), an offbeat Canadian science fiction horror film, prisoners in a mysterious jail travel through rooms rigged with traps.

The extended “Saw” franchise — the 11th film is scheduled to come out next year — has not always stuck to the ideas put forward by the first movie. Wan never directed another “Saw” movie, while Whannell contributed to “Saw II” and wrote the screenplay for “Saw III.”

In the decades since “Saw” was released, Wan has directed tentpole franchise entries like “Aquaman” (2018) and “Furious 7” (2015). Whannell has also settled into the director’s chair, including “The Invisible Man” (2020) starring Elisabeth Moss and the upcoming “Wolf Man,” both modern incarnations of classic horror stories.

That success all goes back to the two friends as fledgling filmmakers trying to find a story that felt compelling, and then landing on ethical choices with bloody results.

Whannell is proudest of the “Saw” franchise’s outsize impact on popular culture. His favorite: In “The Sopranos,” a character becomes fixated on pitching a movie that he describes as “Saw” meets “The Godfather Part II.” Billy the Puppet was recently added to the popular battle royale video game Fortnite.

“Whether we wanted to or not,” Whannell said, “we basically created the millennial Freddy Krueger.”