Actually, that sounds like a fun adventure. I mean fade to black on the activities that led up to the magic shackle key being lost, but beyond that, a fun premise for a basic find-the-macguffin adventure.
See, not every adventure has to have the survival of the world at stake.
My main problem with such an adventure in 5e would be how many players would just short circuit the entire adventure by cutting off the shackled person's hand and casting greater restoration. Would certainly work best as a low level adventure.
Of course, not appropriate for WotC to publish given the wide age range they market to, but I'm not above stealing this idea and putting it in my campaign. Though, I'm running Warhammer Fantasy, so the couple the party is helping out of this "bind" (puerile chortle) would certainly turn out to be secret followers of Slaanesh.