As beautiful young co-stars in the long-running, Superman-prequel TV series “Smallville,” their biggest problem was dealing with their odd young pal, Clark Kent.

Then Kristin Kreuk and Allison Mack signed up for an upstate seminar run by self-help guru Keith Raniere, and they met a real-life super villain.

Raniere was arrested while on the lam in his $10,000-a-week Mexican villa this week, charged by federal prosecutors with running a violent sex-slave cult called NXIVM (pronounced nexium).

Brunette Kreuk, who portrayed the young ­Superman’s love interest, Lana Lang, signed up, but got out before female acolytes began getting forcibly branded with cauterizing pens.

“Kreuk had come first, sometime around late 2005, early 2006,” said Frank Parlato, who first broke the story, in June, of women being forcibly branded as part of the ­NXIVM “sorority.”

“Kreuk brought in Allison” soon afterward, said Parlato, who worked as the group’s publicist in 2007 and 2008.

Mack played Clark Kent’s starry-eyed blond pursuer, Chloe Sullivan.

“Lana” and “Chloe” went from starring in the popular, sugar sweet CW series to starring as ­Raniere’s top sex-slave recruiters, said Parlato.

“Allison was used, as was Kristen, as a lure to bring in other women because of their celebrity status,” he said.

Kreuk severed ties with the group in 2012, soon ­after the Albany Times Union wrote of allegations that Raniere had sex with underage girls.

But Mack stayed on and, according to Parlato, helped come up with the idea of sizzling the “brand” into initiates’ skin, near their groins, as they were held down. About 2-by-2-inches, the mark combined Raniere’s initials, KR, with Mack’s, AM.

“Kreuk was also in the inner circle, but branding was a later addition to Raniere’s abuse of women,” Parlato said.

An emaciated and frenzied-looking Mack can be seen in cellphone footage, obtained by Parlato, of ­Raniere’s arrest.

“Allison Mack expected to be arrested next,” he wrote Tuesday in artvoice.com, saying she is an unnamed co-conspirator in the Raniere case.

“She started as a slave and she became a slave master,” Parlato claims. “Her nickname among defectors is ‘Pimp Mack.’”

Spokespeople for Kreuk and Mack did not respond to requests for comment.

This article originally appeared in the New York Post.