In Un Show Mas: La Pelicula everything begins with a special ship that comes from the future. This ship lands right at the feet of Mordecai and Rigby, characters that we know by the series of animation. These two characters hallucinate with what is happening to them and more when the ship opens.
A space warrior comes down from the ship, not coming to harm them, but to warn them, a warning. He tells them that a time will come, which is nothing more than a tornado that alters time.
This tornado creates what scientists call wormholes, which gobble up time and space. Thus, the appearance of time means that the Universe is in danger of disappearing altogether.
The two friends are the only ones who can save the Universe, so they will undertake the best work of their life, facing all kinds of enemies, dangerous and grotesque at the same time.