I feel as if there’s a pop-culture reference here that I’m just not getting. Want to offer any hints?

There is not a reference of which I am aware. Save, of course, for the old trope of some sinister thing being kept under lock & key until something bad happens.

Interesting. So, unlike a Seinfeldian “show about nothing,” this is a “story about anything.” The key represents some sort of catalyst for a disaster; the chain/line of people handing the key down to the final (”right”) person represents a series of human errors compounding the problem (???), and the “all hell breaking loose” could be, well, anything big and bad and massively tragic.

Now I’m thinking more of “The China Syndrome,” for some weird reason: chains of events that lead, through human error, to the potential for mega-disaster.

Indeed. This could be a story about anything.

I didn’t indend human error to be involved in this story. 721 will be intentionally unlocked by the right person. And the outcome of that action will be unpleasant in Jackson’s estimation. The scale of the unpleasantness is entirely the result of the reader’s interpretation of the story.