It's the reason, 40-odd years later, that he'll be here next weekend, talking about unanswered questions in a free presentation in the Bloomington Public Library Community Room (2 p.m. June 28).
The impacting moment occurred when Fisk was around 10 years old.
It happened at his rural home in Northern Wisconsin, just outside the window he happened to be gazing through.
"What I saw was a silver, cigar-shaped object moving across the sky ... in broad daylight," he still vividly recalls, lo these many decades later.
"It wasn't a balloon. Or an airplane. Or a helicopter."
Fisk's siblings were outside, "playing at the end of the driveway." He sprinted down to them, yelling all the way, "Look at the sky!"
They looked. They saw.
It was a classic Baby Boomer tableau, worthy of Spielberg: A band of all-American kids gazing heavenward, eyes and mouths ajar, at something they were certain was not of this Earth.
"We watched it quite awhile," Fisk continues, "as it moved slowly and silently across the sky, then disappeared behind some trees on the horizon."
Capping the Spielbergian moment: "Then we heard the sound of a helicopter. And then we saw it: a black military 'copter following the object in the same flight path."
Shades of "Close Encounters" and its Devil's Tower military/UFO aerial maneuvers?
"Were were miles from any airport," he says. "There was no military base in the area. And it was the one and only time we ever saw a helicopter fly over that area."
The first of many deductions/theories that Terry Fisk would offer over the course of his life ensued: "Because it was following the same path (as the UFO), I was convinced it was following it on purpose."
The story, in that pre-net, pre-social media, pre-24/7-news era, remained pretty much a Fisk Family legend ... more rural than urban.
"No one took what we said we saw seriously," he says today, still waiting for the rational explanation that would scratch his lifetime itch.
"We were just kids."
No more, though.
As a grown man, around 10 years ago, he faced his second major confrontation with the unknown ... no less life-altering, but far more earthbound.
"My brother (Rick) and I were in a cemetery where our great-grandparents are buried," he recalls. "I asked him if he would take a picture of me standing next to the headstone."
"When we uploaded the pictures and started going through them, they were all fine ... except one of me standing next to the headstone with a strange mist surrounding me."
Shades of the silver cigar-shaped object years earlier: there was no rational explanation to ease him out of the situation.
"It was a bright, clear, sunny day, with no mist," he says. "There was no smoke in the air. So I took the photo around to different people, including camera and photo experts, trying to find a natural explanation. No one had an answer."
Until someone offers a rational diagnosis, he's assuming the mist was not of this Earth, or at least that sector that says ghosts don't exist.
The episode eventually brought him in contact with a paranormal investigator named Chad Lewis, who would eventually become his writing partner on a series of state-themed books, including "The Illinois Road Guide to Haunted Locations."
They joined forces to found Unexplained Research LLC, based in Eau Claire, Wis., which, among other endeavors, publishes the book series and stages conferences.
Fisk's first appearance at the Bloomington Public Library last fall was devoted to the kind of thing he experienced at his great-grandparents' grave, and was wildly popular, a library spokeswoman said,
For next weekend's encore presentation, Fisk will hark back to his childhood experience with a UFO, focusing on that sector of the unexplained as it relates to Illinois, which, he says, ranks No. 6 on the list of states with the most sightings (see accompanying story for a sampling of Pantagraph accounts over the decades).
"While the bulk of the investigations we've done have been regarding ghosts, I really do find the UFO reports among the most intriguing," says Fisk.
He says that the sightings tend to go in cycles, or what experts call "UFO flaps," which are three-and-four-year periods when there is a wealth of sightings, "followed by a tapering off ... and right now, we're in a tapering-off period."
Even so, "there are still a lot more sightings than people realize."
Most aren't reported in newspapers or the media, he adds, because the witnesses are reluctant to risk ridicule ... particularly in this age of easy digital manipulation where, Fisk notes, "a photo or video of a ghost or a UFO are basically useless."
The gold standard for today's UFO reportage, he says, is the mass sighting, when large groups of people aim their phones and cameras at the same unexplained object.
The most famous case hereabouts, and which will be discussed in Fisk's presentation, is the so-called "Tinley Park Lights" episode of 2004.
Passing over the Chicago suburb was a triangular object with lights at each point, flying low and witnessed by many.
"That was a case where having so many people walking around with the capacity to take photos and videos was an advantage," he says.
"There were hundreds of eyewitnesses and dozens of people filming the UFO from different perspectives. So researchers were able to gather up all the videos, which were time-stamped, and examine the same object from different angles."
It's a far different world on those terms than it was 40-odd years ago when Terry Fisk had his backyard encounter, when all he had for proof was what he observed with his eyes.
Then as now, though, "the vast majority of these sightings are the result of misidentification, and some are hoaxes."
"But there is that small percentage that cannot be explained." he adds. "That's what has always really intrigued me ... and continues to."