When Spike bashed the back of our heroine against a bath, Buffy fans weren't expecting it. Our hearts were in our throats as he continued to derobe her, pulling her across the floor and telling her "let yourself feel it".

The episode aired in 2002, around the time people first began taking to the internet to have proper, nuanced conversations with strangers about their fandom. Viewers – particularly women and victims of sexual assault – boycotted the show in their masses. They were triggered by its attempt to show how rape happens in a step-by-step way that felt completely out of tone with the rest of the show. Buffy was supposed to be our feminist hero, and here she was, for the first time in the six seasons we'd spent with her, struggling in a situation that wasn't just male monster vs female slayer. There was no metaphor for human evil, as was usually the case in Buffy; no greater evil. This was just a man forcefully demanding sex from a woman in a very darkly human situation.

Spike never manages to rape Buffy – she eventually wrestles free. But the damage is done. "Seeing Red" – which aired 15 years ago today – became infamous not just for those who watched the show, but for anyone interested in TV or pop culture at large. This single episode set the standard for the way in which we talk about TV today.

"For TV scholars, Buffy was the birth of what we now call 'quality television', in terms of shows that hit certain characteristics and that we can discuss as a quality text," Buffy Studies scholar Lorna Jowett tells me. "We learned how to talk about television as an art form from this show. We're still talking about whether it's feminist many years later."

That scene sent me into therapy. When anyone watches Buffy, they are Buffy – James Marsters, Spike

"Historical context is everything, and scenes like this provoke a lot of debate in my class because TV is a niche, targeted thing, and it's always moving forward," Jowett says.

It's just as uncomfortable to watch now as it was then. Buffy fans I know prefer to pretend that the episode isn't canon, or just deny its existence altogether. What made it so frustrating, so emotionally messy and, for many, utterly irresponsible, was that Spike had been built up as a three-dimensional man by Joss Whedon and actor James Marsters. He was one of the characters we loved the most. He was a blood-sucking sex symbol with a heart.

The show and the episode's defenders say the scene was justified in the context of the story arc. Prior to the assault, Buffy and Spike had been sleeping together secretly, much to Buffy's shame. From the first moment we meet Spike, we've known that he will do anything for the woman he loves – that he's very capable of being driven to obsession and revenge. Leading up to this scene, Spike has been scorned by Buffy and has lashed out by sleeping with one of her friends. Buffy, understandably, is disgusted by Spike's behaviour.

Yet, when we should have been free to abandon Spike – as a modern audience would after such a betrayal – we were manipulated to feel we couldn't. Joss Whedon actually rewrote a later scene to make it clear to an audience feeling weird about Spike that Buffy was using Spike for sex during the period leading up to the assault. Season seven was a joy for anyone harbouring a secret hope for Spuffy, as we saw Spike work for Buffy's love, which is eventually reciprocated in the final moments of the show, when she tells him she loves him.

All, it seemed, was forgiven and forgotten. The audience was convinced to make amends with the character, to make excuses for his behaviour.

For Marsters, the filming of that scene was "the worst day of [his] professional life and one of the worst of [his] personal life".

"I remember I went up to the person that actually wrote the script and said, 'You writers, you don't really understand what you put us through. You write this, but we have to live it,'" Marsters tells me over the phone. "I had whiplash from an old Judo injury when I was younger, and when I get too tense it just snaps and it's painful. I remember just the very first shot, where Spike kind of opens the door and steps in the room, I got about one-and-a-half lines out and there was a pop. I was like a marionette with the strings sliced, and I just fell to the ground."

"We did traumatise the audience maybe a little too much. I'll now routinely turn down roles with rape scenes in them" – James Marsters, Spike

"What I will say is that that scene sent me into therapy, which turned out to be a very good thing for me," he admitted. Rewatching the scene after our conversation, I can see it in his face; the hesitation – the resistance – doesn't feel quite as much Spike's as Marsters'.

The actor is acutely aware of why it hit such a nerve. "Storytelling is a vicarious experience, so the way you as the audience member get the experience is by climbing behind the eyes of the lead. So when anyone watches Buffy, they are Buffy."

"Whether you watch it out of context or in context, it's still really hard to watch," says Lorna. "But I think you have to have that emotional investment in the characters to really feel it."

"It was keeping with the way that the writing had been going the whole time, so I think it was a worthy risk. But it very nearly blew up in all of our faces – we just barely hung on after that," Marsters remembers. "We did traumatise the audience maybe a little too much. I'll now routinely turn down roles with rape scenes in them. Not again."

For this scene to be held by fans as the biggest misstep of the show – worse than the creation of much hated whiner Dawn, more terrible still than the episode where a perverse haunted house forced Buffy and Riley to have continuous sex – is significant. But if you want to look at moments in TV history as a yardstick for change, there are few as debatable for a culture scholar, feminist or pop culture fanatic than this.