While exploring the Everfree, Twilight finds a mysterious refrigeration machine with a can of some kind of ice cold beverage waiting in its receptacle. Wishing to study the machine more, she teleports the can back to her castle as she isn't thirsty, but quickly loses interest as the machine's buttons do not appear to work.
A short time later, another one of those 'totally sealed away threats to the existence of Equestria' type problems reappears - the Rainbow of Darkness - and before anypony can do anything it quickly overwhelms several cities with Ponyville next in its path. The now exhausted Twilight, having no idea how to stop the evil rainbow, decides to have a drink and unwittingly summons the best chance Equestria has of salvation.