Im glad you asked. Basically Diamond Tiara rules Equestria and has turned Ponyville into her own sparkly domain. She enslaved the Cutie Mark Crusaders and the other fillies. She says the fate of Equestria lies in the hooves of the " blank flanks". Babs Seed comes to warn Diamond Tiara that if she keeps corrupting Equestria like this, all of ponykind will go extinct. Diamond Tiara simply kicks her out of her castle.
Its up to Rainbow Dash and friends to help save the world! RD gets a message from Princess Celestia saying that Diamond Tiara is to blame for her weather control being bent out of whack. It also turns out Diamond Tiara rules Cloudsdale, the place where all the water has to be funneled through a tornado made by pegasi to get to the rest of Equestria. Diamond Tiara is screwing this up somehow, and Rainbow Dash is the only pony who can stop her from " driving all ponykind to extinction".
Seriously, Princess Celestia basically tells Rainbow Dash that is she fails, everypony will die.
When RD gets to Cloudsdale, she finds all the other pegasi are enslaved too, but she cant save them because theyre being guarded by killer mecha ponies! These robot ponies" are all programmed to say " Bump, bump sugar lump rump!" every so often. These robots are referred to as " Robonies".
After RD's first attempt to save the pegasi fails, she sees the rest of the Mane 6 in their balloon, there to help her. We also see this Gundam reject that cant believe Celestia chose a MARE to save Equestria from extinction. Turns out its just Pinkie Pie being Pinkie Pie. Then Twilight Sparkle explains how the Robonies have hypnotized everypony into working for the crazy Diamond Tiara.
When the Mane 6 get down to Ponyville, they boldly walk up to the steps of Diamond Tiara's castle; Diamond Tiara sees them and has Sweetie Belle cast a spell to blow them away, but Twilight's magic prevails. They find it odd Sweetie Belle had such a capability, being a filly. It turns out Diamond Tiara is wearing the Alicorn Amulet and only she can take it off. The Mane 6 get thrown in some weird trap and they have to work together to get out.
To make a long plot short, Rainbow Dash blows up Diamond Tiaras castle, and everything goes back to normal, and Pinkie Pie hosts a celebration at Twilight's place.