The 19-year-old beauty vlogger Marina Joyce specializes in glittery, achingly upbeat fashion and makeup tutorials. But lately, fans say, her YouTube and YouNow clips have taken a darker turn: In her last YouTube video, “DATE OUTFIT IDEAS,” the petite blonde seems almost drugged, speaking with an unfocused, half-smiling expression and doing slow, languid turns in front of the camera.

In any other realm of popular celebrity, such a misstep might provoke a couple of tabloid headlines or inclusion in some unflattering magazine photo collage. But this is YouTube, where fans feel an almost cultish devotion to their stars. Hence, within hours, Joyce’s fans had placed multiple calls to local police and Scotland Yard, trended the hashtag #savemarinajoyce with well over 700,000 tweets, and plastered Joyce’s social channels with absurd conspiracies — ranging from “she’s being forced to film by an abusive boyfriend” to “she’s been kidnapped by the Islamic State.”

“Guys, you’re worrying my friends,” a frustrated Joyce said in a YouNow livestream, immediately after getting off the phone with a caller who appears to ask if she’s all right. “I can’t tell you what happened, but I am okay and I really do love you guys.”

One of Joyce’s friends, the YouTuber Karim Slimani, was a little more explicit: “Someone’s PERSONAL LIFE AND PROBLEMS ARE NOT URS TO SHARE THIS ISN’T A F—ING TV SHOW,” he tweeted.

Suffice it to say, Joyce has not been kidnapped — by the Islamic State, the man who recently appeared in her Instagram, the shooter of the “Date Outfit” video, or anybody else. Local police have tweeted that they visited Joyce’s home and found her in good condition. Several of her friends, including one who spoke directly to the Post by phone, have attributed Joyce’s appearance to personal problems. (Joyce has publicly discussed past alcohol abuse, and claims she was assaulted by another YouTube star at an event three years ago. She did not return the Post’s request for comment.)

Whatever the exact issue, though, the reaction has been truly bizarre. In Joyce’s YouTube comments, as well as on Twitter, Facebook and forums like Pretty Ugly Little Liars, self-appointed sleuths have embarked on a thorough forensic analysis of the YouTuber’s recent videos. The backs of her arms are bruised in “Date Outfit,” they point out. They claim her blinking almost spells SOS in Morse-code. If you listen with the volume turned all the way up, there’s a rustling noise at the 13-second mark that could be heard as “help me.” If you fudge it a little.

In another recent video, “Question and Answer,” Joyce seems near tears explaining herself to fans who say she’s changed. Conspiracists insist that she’s because she’s been kidnapped and her captor is in the background, forcing her to tell her viewers that she’s okay.

The week before that video, Joyce posted an “Every Day Makeup Tutorial” rife with inconsistencies and jump cuts. That’s been taken as evidence that Joyce is dead and someone’s covering the crime with mashed-up videos spliced from Joyce’s other shots.

Joyce’s fans haven’t just stopped at idle gossip and analysis, though — they’ve also taken action. One vigilante, posting under an Anonymous-affiliated Twitter handle, began placing calls to British police departments and live-streaming from a site not far from where Joyce lives. At one point, the account tweeted a picture of a man cropped from Joyce’s Instagram and suggested “put[ting] a hit out” on him. Other Twitter users circulated photos of Joyce’s mother and of a man allegedly reflected in Joyce’s pupil, and demanded to know the identity of the man filming the “Date Outfit” video.

“It’s a common thing for misinformation to be spread on the Internet,” said Slimani — the filmographer in question. “In this case, it’s just a person speaking a worry in her fans, and those fans escalating the situation. That could happen with any form of content … It’s just people being human.”

But maybe it’s actually a bit more than that: Maybe it’s people being human on YouTube, a place that complicates our usual notions about celebrity, privacy and distance. Research shows that young people often feel a sense of intimacy with YouTube stars that they don’t feel with actors or musicians. People like Joyce film in their bedrooms or bathrooms, after all; they respond obsessively to viewer comments and questions. In Joyce’s YouNow stream, she also reassured fans, repeatedly, that she “really, really loves” them.

It almost follows that her fans should feel psychotically protective; possessive, even, as if they have a right to know exactly what’s going on in every aspect of her life. Social scientists have a name for this type of relationship: It’s called a “parasocial interaction” — the one-sided illusion of intimacy on the part of a person who knows a lot about someone else — and it’s in full-force online.

Joyce’s fans don’t actually know her or her issues, but they truly believe they do. And those illusions can be powerful — dangerously so. Several of her friends and fellow YouTubers report being harassed by fans who accuse them of covering up the “abuse.”

Joyce is at home right now, a friend told the Post; she’s avoiding the media who have gathered near her house, and has spoken to the officers that Twitter sent. In a Facebook chat Slimani posted to Twitter, Joyce’s mother said the vlogger was “a bit vulnerable” but coping well with the attention.

“People on YouTube can get so confus[ed],” she wrote, in what may be the clearest distillation of this whole mess. People on YouTube get confused, all right: both about what’s going on, and what their role should be in it.

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