Sevii Island 4: Floe Island
You've obtained the Ruby Plate, so now you need the Sapphire Plate? We'll get to moving there, but first you need to be here. There's a few things to do in this small town. There's a Daycare here, where you may breed your Pok?mon by leaving two at the Daycare. Lorelei also lives here, but is missing from her home, and isn't at the Elite Four (as it is closed) either. Go to Icefall Cave, just at the top-right corner of Four Island. Browse around, as there is an Item Ball containing the HM 07 Waterfall Disc. Then, go up the big waterfall, and you should come across Lorelei, and some Rockets. You will be thrusted into battle, so go and defeat the Rockets. They will disappear, and Lorelei will thank you and return to her home.