WARNING: The following contains major spoilers for Season 1 of Katla, now streaming on Netflix.
In Netflix's Katla, it takes a while for Season 1 to reveal what's up with the mysterious Icelandic town, Vik. People are emerging from the ash of the titular volcano, but no one knows why these doubles are coming to life. They're taking the forms of people who died, and who're still alive, but one person is brave enough to take things into his own hands: the scientist, Darri (Björn Thors), who goes spelunking to discern the sinister secret behind it all.
Darri's a temporary visitor -- an expert on volcanoes brought in to dissect the pyroclastic flow after a recent eruption. But his world's rocked when he finds his dead son, Mikael, alive and well at the site. He hides the boy for a bit, bathing him and removing all the ash to truly find that this kid knows everything his son knew.
Sadly, Darri senses a darkness within and locks him in a shed as the boy's dredging up bad memories. It's why Darri and his wife, Rakael, broke up, as he couldn't connect with the mini-terror he thought his son was becoming. However, the kid breaks out of the shed and calls Rakael to come pick him up. She steals him away which kicks everything into motion.
She also senses hatred in him and leaves him in the wilderness, returning to Darri to figure out what the hell's going on. The innkeeper admits that with other doubles coming to town, while it's not scientifically proven, it may be changelings. Vik's folklore spoke of people coming back to life centuries ago, giving their loved ones a second chance and proving death wasn't a finality. In some cases, the returned ended up killing their beloved, so it does seem that they go mad -- or in the case of others who've disappeared, they served a purpose and left.
Darri refuses to believe it's something mystical so he goes into the glacier near Katla. As he scours the cave, he does see bodies stuck in the muck, which explains why they come up to the surface blackened like this. When he gets back, however, as he dissects material taken from the site, Darri uncovers elements revealing a meteorite. Using his geographical knowledge, he deduces a meteor crashed here in the prehistoric era and poisoned the land. It gave rise to Katla, who's been shooting out infected ash since. It's why the townsfolk who were exposed, as well as a visitor like Darri, are seeing people they grieved for emerge again.
With the likes of Ása coming back, it's down to her family missing her after she died. When Darri works with her sister, Gríma, the rest of the puzzle becomes clear. Gríma's sister is more despondent now and Darri realizes the last impression they have of people is how they'll return. In his mind, his kid -- who was bullied and tried to burn a school down before committing suicide -- was a monster, ergo why this Mikael manifests as such. It's proven when an old couple pick Mikael up but he slits the wife's throat with a knife and causes the car to crash. He escapes, leaving them dead as he hated how they lied about whether they'd take him to child services or to his mom.
When Darri hears more about Gríma's past and the differences between her sister's two selves, he concludes that it's not just the final impression but the way they think about people in general that'll form this new person. Gríma just remembered how sad her sister was after their dad's cheating and their mom's suicide, so that image -- not her as a rebel -- is what created the new Ása. Reconciling what the innkeeper said, while their thoughts manifest through this volcano, Chosen Ones who come back usually are there to help the people who conjured them, but there's always a chance, per the folklore, they are here for revenge.
Created by Sigurjón Kjartansson and Baltasar Kormákur, Katla stars Íris Tanja Flygenring, Ingvar Sigurdsson, Aliette Opheim, Þorsteinn Bachmann, Haraldur Stefansson, Sólveig Arnarsdóttir, Baltasar Breki Samper, Birgitta Birgisdóttir and Björn Thors. Season 1 is now streaming on Netflix.