Six objects older than the universe, each one controlling a single aspect of existence, scattered and hidden until somebody tried to collect them all.
The Stones are the spine of the first twenty-two films. They arrive one at a time, usually disguised as something else, and for most of that run nobody on screen understands what they are looking at.
Their real function is structural. They gave a decade of separate films a reason to be one story, and they let a villain who appears for thirty seconds in 2012 remain a credible threat until 2018 without doing anything.
Show detailsWhy the vessels matter more than the stones
Almost none of them are found as a bare stone. They turn up as a cube, a sceptre, a substance, an orb, an amulet. That disguise is doing two jobs: it lets a film use one as a plot device without explaining the whole system, and it means a rewatch shows you a Stone in a film you thought had nothing to do with any of this.