The Most Dangerous Animal of All Chronicles One Man’s Mission to Prove His Father Was the Zodiac Killer

On its surface, The Most Dangerous Animal of All, FX’s first true crime documentary series, is a revisitation of the case of the Zodiac killer—one of the most notorious unsolved mysteries in American history. But over four episodes, executive producer Ross Dinerstein and director and executive producer Kief Davidson show that their real fascination lies with Gary Stewart, a man who is so convinced his biological father was the infamous serial killer that he’ll go to great lengths to prove he’s right, despite shaky evidence.

The first part of the series, which debuts March 6 and will be available on Hulu the following day, is anchored by Stewart, who co-wrote the bestselling book from which the documentary series takes its name, with the journalist and true crime writer Susan Mustafa. The series covers details revealed in the book, recounting how Stewart was adopted and grew up with severe abandonment issues that continue to drive his restless search for his origins and identity.

Stewart got some answers about his background in 2002, when his birth mother Jude Gilford reconnected with him and revealed that his father was a man named Earl Van Best Jr. As far as origin stories go, this one is disturbing enough: as a 27-year-old, Van Best met Gilford when she was about 14, and began an illegal relationship with the minor that landed him in jail.

Upon his release, Van Best went on the run with Gilford, ending up in Louisiana, where Stewart was born. (Their relationship received media coverage at the time and was dubbed the “Ice Cream Romance” in San Francisco newspapers). Van Best abandoned his son in the stairwell of an apartment complex when he was just one month old in the early 1960s, Stewart says in the series. He was eventually arrested and sent to a mental hospital in California, and Gilford put the baby up for adoption, according to the series. Van Best died in 1984.

The revelations were enough to confirm that Van Best was far from an ideal father figure. But Stewart, unable to shake his growing obsession with configuring his own identity, is certain there’s more to know. The more he’s dug into the story, the series shows, Stewart has convinced himself that he’s found several links to show that Van Best was the Zodiac killer. The evidence is circumstantial at best—Stewart claims his father’s mugshot resembled the police sketch of the Zodiac killer and that the men had similar handwriting. Other details stick for Stewart, like knowing both the Zodiac killer and Van Best enjoyed ciphers and shared an obsession with school buses. But none of his points are solid enough for authorities to resolve the case.

Although the documentary is based on Stewart’s book and allows Stewart to present his side of the story through dramatic narration, the filmmakers are careful to fact-check his claims, which also received pushback in online communities and forums dedicated to finding the Zodiac killer. Through the work of an independent investigator, the final episode of the series show holes in Stewart’s assertions and reveals mistakes that were published in the book, much to Mustafa’s shock and devastation. In one example, the filmmakers reveal that Stewart fabricated information that he said came from a police report. After learning that major parts of the book were based on incorrect information, Mustafa says Stewart “had to be able to create the worst possible person in the world to be able to explain to himself how a father who’s supposed to love his son could just leave him like that on a stairwell.”

Ultimately, The Most Dangerous Animal of All examines how Stewart’s quest for his true identity never fully satisfies him, and comes at the cost of several relationships, including multiple marriages. In seeking to establish that Van Best was the Zodiac killer, Stewart loses sight of the overall truth: that his father was likely just another bad man. Still, sharing his staunch belief that he’s the son of the Zodiac killer has earned Stewart widespread attention—even if it hasn’t filled the void he continues to feel. He sums it up well himself when he says, “I could continue this journey for another 10 years and never, never, never feel any better about me.”