Durand-Durand forces Barbarella at gunpoint to take the invisible key she wears in a chain around her neck to the bedchamber of the Black Queen, also known as the Chamber of Dreams. There they encounter an invisible wall and have a difficult time trying to discern the location of the keyhole. Luckily, in a struggle she drops the key, and it falls through the transparent floor which ripples like water under their feet. This unlocks the invisible door and allows them both to pass into the Chamber of Dreams.
Just within the chamber atop a pedestal sits a second invisible key that can reclose the invisible door. To imprison Barbarella and the Queen within, he rushes in, grabs the key, and throws it down to the floor before Barbarella can react.
Though of course this sequence of events is in place simply to show that Durand-Durand has imprisoned Barbarella and the Black Queen, as a system it raises many questions.
An invisible key certainly means that it can be hidden in plain sight and so has some extra security from that perspective. But its being invisible means that recovering it when lost is problematic at best. Plus, unless it is kept somewhere on the body, the invisibility places a burden on the memory of the keeper as to where it is. (You can’t leave a physical reminder of where it is or you lose the benefit of its being invisible.) Are these costs to memory worth the mildly increased security?
Also, as we see, any spot on the floor is an acceptable target for dropping a key. At first this might seem like hyper-usability, since it’s nearly impossible to miss the keyhole, but it also means it’s hard to recover from a mistake. No, wait. It’s impossible to recover from a mistake. That is, if you fumble and accidentally drop a key, the door will activate. We don’t see a key-return mechanism, so this mistake is deeply unrecoverable. Even if that key-return mechanism is somewhere else in the palace, that’s a disaster for usability.
That might be bad enough, but when you realize that this is a royal chamber, it seems an impossible oversight, as if it were custom designed just to imprison people. The Queen seems genuinely distressed when she realizes Durand-Durand has stolen her key, insisting that they “are doomed. Dooooomed!’ but I’m pretty sure anyone who had given it just a moment’s thought before would have realized that this was the inevitable result of this ridiculous design. Maybe the true power of the Mathmos is to keep the queen perpetually blind to stupid interaction design.