Sarah Farrell Mackessy, co-owner of Lille Aeske, fielded a frantic call at the Boulder Creek gallery a couple of weeks ago. The woman on the other end of the line told her, “‘I need to get to your show tonight. I have no idea where you are.”

It turns out the woman had received a mysterious email from a friend, who wrote, “I went to this show. I don’t even know how to describe it to you, but you need to go.”

The show was Spektrum, the live, interactive installation that Mackessy and her husband James, who run the gallery together, have orchestrated for the last few weeks at Lille Aeske. And while that email might seem bizarre, I totally understand it—because that’s exactly how I had to describe Spektrum to my friends. “I can’t tell you what it is, just go,” “I guarantee you’ve never experienced anything quite like it” and “it will blow your mind” are a few of the things I remember saying to people I knew would be up for it. Clearly, I wasn’t the only one.

“The news about it traveled so quickly,” says Sarah. “We were worried that no one would come; we were like ‘Who’s going to come to this?’ Because we couldn’t even really find a way to talk about it in the beginning.”

That was the most frustrating part for those of us who went, too. Georgia Johnson of GT, who wrote a review of Spektrum after its first week, was tearing her hair out trying to figure out how to explain the experience without, you know, explaining the experience. Nobody wanted to spoil anything, because there was a certain purity to the whole presentation that seemed like it could be ruined by knowing too much about what to expect.

Now that Spektrum has finished its run, though, it seems like a good time to marvel over what the Mackessys and their collaborators were able to accomplish. One caveat: the Lille Aeske owners tell me that because of its success—it was sold out every week after the first weekend, and demand was so high by the end that they bent their own rules to allow for 16 visitors instead of 12 on the last night—that they are thinking about bringing the installation back in some form in 2019, so maybe spoiler alert.

The best way I can think of to sum up Spektrum is that it was a solo journey that turned the basic concept of a gallery on its head—rather than looking outward at objects of art, the people moving through the installation arguably became the objects themselves, with each subsequent room in the hour-long experience encouraging the visitor to look inward in a different way.

The first room was a very retro kitchen in which a masked man (actually James) moved about in an apron, doing what would have once been considered “motherly” type things: dusting, washing dishes, looking at photographs, sitting with me in silence at a small table (visitors were asked not to talk, as it would have surely ruined the quiet, solitary ambience for those who were in rooms in front of or behind them). This was also where the first of a series of letters was laid out on the table—from an unnamed person, inquiring about my well-being and describing memories.

As more letters appeared in each room, the themes of Spektrum started to emerge, stimulating questions like: Where do I come from? Who are my ancestors? What is my place in nature? These were developed through each room—one where I was invited to play an extremely cosmic piano, another where I was asked to meditate as empty frames teased the question of who my mind would imagine in them—until the climactic set piece, where an eerily lit bed that seemed ominously like a deathbed awaited. A veiled figure (actually Sarah) invited me to lie down on it, and when I did, I saw that there was a screen embedded into the top canopy that flickered in front of my eyes like footage of memories. Again, the theme of nature and especially big trees kept recurring in the images.

“One of the things we considered was what we love about being here, and the feel of this place,” says James. “As we were collaborating on the piece, we all agreed that the presence of those trees—as mentioned in the letters, they have a timelessness, a possible immortality, a lineage and an ancestry that is so visible and is such a force here—seemed like a very rich vein to tap into.”

One of the rooms featured a guest artist that changed every week, and each one did something completely different there. The week I went, the interaction with the artist was particularly personal and direct; the role she played was a sort of “Mother Nature” that tied in well to the other themes of the installation—and provoked some of the most emotional reactions of the entire run, Sarah says. But regardless of what week they went, visitors knew they were experiencing something unique.

“I think one common point of feedback was just how considered and special and thought-of they felt as they moved through the installation,” James says. “They felt like they were being taken care of. That was sort of built into the design of the show. You go through individually, and that immediately eliminates any sense of group dynamics or audience dynamics. The whole show is literally for you at that point. I think it’s rare to feel that kind of thing these days.”

At times, the whole thing was like a whirlwind for the Mackessys and their collaborators—and eventually, like a marathon. But the success of Spektrum and the positive feedback they’ve gotten has them thinking about what other kind of outside-the-box installations they can do next year.

“The challenge was that this is our space that we converted for this installation, and we quite literally immersed ourselves in it, day and night. That was a little maddening at times,” says Sarah. “So there were definitely peaks and valleys to it. But we’re sitting in it now, getting ready to de-install it, and are quite sad that we have to change it back.”