Batman fails to stop the Joker from buying national air time and televising his newest scheme - he's planted bombs through Las Vegas and dares the Justice League to stop him. The first bomb is a dud but the League soon find the Joker has help - he's created his Royal Flush Gang of five super-powered individuals with powers to rival the League.
After a initial fight which proves inconclusive, the Leaguers split up to find the rest of the bombs - Batman and Jack battle it out at one site while Ten and Superman fight at another. Hawkgirl and Green Lantern join forces and take out Queen but with seconds to go to stop the bomb, Green Lantern shoves Hawkgirl out of the building just as the explosion goes off.
The Royal Flush Gang
The Royal Flush Gang here are a unique creation - five super-powered children who the Joker takes in and uses as his henchman after giving them a card based motif. Ten is super-strong and doesn't feel pain; Jack can stretch his body; Queen can manipulate metal magnetically; King is a pyrokinetic; and Ace can drive people insane.
The Gang here bears no resemblence or apparent connection to the generations-spawning Royal Flush Gang crime family of Batman Beyond who appear in the "future" (starting with the episode "Dead Man's Hand"). However, it's certainly possible the Joker either knows of the BB RF Gang and created his own to mock them, or created his own purely coincidentally.
In the comics the Royal Flush Gang bears no resemblence to either animated version - it was brought together by Amos Fortune (Justice League of America #11 - May 1962) and in have gone through several incarnation having a vast array of technological powers. In both the comics and the Super Friends series, the Joker also formed at least one incarnation of the Royal Flush Gang as well.
Harley Quinn is a former psychiatrist who was assigned to treat the Joker and ended up falling love with him. To display her love for her "Puddin'" she donned a harlequin's outfit and adapted a similar motif. At best the Joker seems to consider her a disposable annoyance with the occasional use, but Harley keeps coming back for more. In the WB animated universe, Harley Quinn first appeared in Batman: The Animated Series, in the episode "Joker's Favor." She has since proven to be a popular spinoff character and has had her own comic book series. This is her first appearance in Justice League.