3. Red Matter
A plot device is an object or character in a story whose sole purpose is to advance the plot of the story, or alternatively to overcome some difficulty in the plot. In Star Trek, that was the deadly Red Matter. It was the space equivalent of a Swiss Army knife, it did everything you needed to at any given moment, it's just a shame that Orci & Kurtzman couldn't be bothered to give it a proper name. A small drop of the substance creates an artificial black hole that consumes everything around it. No mention of what happens to the black hole after it has finished doing what you want it to do, it's lucky it didn't devourer Vulcan's brand new moon, Delta Vega. But the fun with Red Matter doesn't stop there, it also acts as a time travel device, you can fly through the black hole it creates and journey to the past because in Orci & Kurtzman's universe, that is what black holes do. However, make sure you pick the right time to fly through the black hole, if you try doing that at the climax of a movie, the black hole wont send you hurtling through time, instead it will destroy your ship. It's hard to take things like Red Matter seriously, even in science fiction if you don't set its rules and limitations clearly. You can't just make it do whatever conveniently fits into your script, it then move away from being a mysterious substance to be feared and becomes nothing more than a lazy plot device. And if one small drop of red Matter creates a black hole big enough to consume a planet, then the amount used at the end of the film should have made a HUGE black hole. But no, it was just big enough to destroy Nero's ship so we can all relax.