Scott D. Pierce: Hey, true-crime drama fans! What if your father turned out to be a serial killer?

Before you swab your cheek and send your DNA to one of those genetic testing/ancestry services, consider the real-life story of Gary L. Stewart. It will make you worry about what skeletons might be in the family closet.

His search for his biological father led him to a violent pedophile/rapist who abandoned him as an infant. And, as Stewart dug deeper, he became convinced his biological father was a notorious serial killer.

The story of his search and his obsession has been adapted into the true-crime drama “The Most Dangerous Animal of All.” It’s perfect for binging — FX is airing the complete four-part, four-hour series on Friday (7-11 p.m. on Dish and DirecTV, 9 p.m.-1 a.m. on Comcast), and it starts streaming Saturday on Hulu.

“This is a story about a man’s quest for his identity,” said executive producer/director Kief Davidson. “It’s also a film very much about obsession.”

It has the elements that make true-crime drama so perversely popular. Horrific crimes. Mysteries revealed. A possible police cover-up. And controversy.

Like many adoptees, Stewart craved information about his birth parents. When he was 39, his biological mother, Judy Gilford, contacted him. She told him the identity of his biological father — Earl Van Best Jr., who died in 1984. And Stewart learned that, as a baby, he’d been sort of famous because his parents were infamous.

Van Best was a pedophile who, at the age of 27, married and ran off with 14-year-old Gilford. They made headlines when they took off across the country to avoid capture. (The marriage was annulled; Van Best went to prison for statutory rape.)

Van Best dumped Stewart in a Louisiana apartment building stairwell and took off. Then Van Best ended up institutionalized for a time; Gilford decided to place Stewart for adoption; and Stewart was left wondering for almost four decades.

If the story ended there, it would be bad. But Stewart’s sleuthing led him to conclude that Van Best was the Zodiac Killer, who was directly tied to five murders and two attempted murders in northern California in the 1960s and ’70s. The killer — who was never apprehended — claimed 37 victims.

There is unquestionably a strong resemblance between Van Best and a police sketch of the Zodiac Killer. There might be fingerprint and handwriting matches. Maybe even a DNA match. Van Best lived in San Francisco at the time of the killings.

And Stewart turned his investigation into the best-selling book “The Most Dangerous Animal of All: Searching for my Father … and Finding the Zodiac Killer,” on which the TV docudrama is based.

(The title comes from a cryptogram sent by the Zodiac Killer — the only one of four he sent that authorities deciphered. “I like killing people because it is so much fun,” he wrote. “It is more fun than killing wild game in the forest because man is the most dangerous animal of all.”)

The filmmakers hired private investigators who turned up some new evidence, and “connected some dots that people never connected before,” executive producer Ross Dinerstein said. Including the possibility that the police knew more than they were saying.

“I wouldn’t go as far to say that there was a cover-up, per se,” Davidson said, “but there is certainly information that these former detectives were not talking about.”

There is, however, no consensus that Stewart is right. And Part 4 of the documentary turns its focus to Stewart himself, questioning not just his conclusions but his obsession. And Davidson admitted that it was “particularly difficult … to confront Gary.”

It’s not the way a true-crime saga usually ends, which makes “The Most Dangerous Animal of All” that much more compelling.

Davidson praised Stewart for being “brave” enough “to put everything out there and all of his emotions out on the line,” Davidson said. “I wouldn’t have been interested in this story if this was some weird kid sitting in his basement. … This is a successful businessman with family, with kids, with grandkids, seemingly normal, that’s having a hard time with not knowing who he is. And ... there’s a strong emotional story here that needs to be told.”