After realizing that since Allura had a star piece and could invade Equestria at any time, the Mane 6 decide to head back with their new dragon friends to warn the ponies and let them prepare for anything. But, as you'd expect, the Mane 6 didn't get too far when they are summoned on another Unity Quest.

When the Justice League goes missing and villains overrun Metropolis, there's only two teams that can solve this mystery: Scooby-Doo and the gang and the Mane 6! But wait, there's a new dog in town - Krypto - Superman's super-dog with superpowers.

The ponies and Mystery Inc. will need all the help it can get when phantoms menace the Justice League's headquarters. But will they succeed? What's the story with their new pony friend Tempest Nova? And has Lex Luthor finally gone straight? Or is he in cahoots with The Joker and Harley Quinn?

The DC supervillains may have clashed with Superman before, but now they're facing something new; a squad of ponies and a gang of meddling kids!

(S3E4 of G5 Adventures)