After shaking off her chains at Vaes Dothrak, mounting her dragon, and making a rousing speech to the Dothraki, convincing them to join her armies and sail on wooden horses to Westeros, Daenerys returns of Meereen on Game of Thrones. And it's just in the knick of time, since the masters are laying siege to their former city.
Dany's return to Meereen is a vital part of the storyline in Season 6. After achieving her objective of conquering the Dothraki army to add to her impressive military force thus far, Dany needs to unite them all in Meereen. Plus, Yara Greyjoy is on her way with 1,000 ships hijacked from The Iron Islands to merge her fleet and join The Dragon Queen in #LadiesTakeWesteros2k16. This is important because Dany and allllllll her armies are stuck in Essos without ships or any way of actually getting to Westeros to take back The Iron Throne.
Unfortunately, before she can make it back to Meereen, the masters of Slaver's Bay return in their own massive fleet of ships to take the city in retaliation for Daenerys, Breaker of Chains, freeing all their slaves. Tyrion tried to mitigate this problem by telling the masters that, instead of freeing all their slaves all at once, they can phase them out over a seven-year process. Both sides are shifty about this Westerosi dude trying to make foreign policy deals, but they uneasily agree to the terms.
It looks like the masters never had any plans to honor their side of the bargain, however, as their ships approach Meereen and they launch a full-scale naval attack. The city crumbles pretty much instantaneously, but Grey Worm wisely commands The Unsullied to hold the pyramid — the only structure which they can actually defend — instead of going down to the beach to fight on the front lines. He proposes waiting for the masters to approach the pyramid and then fighting them off there, but before Tyrion can counter, a crash is heard above them.
Don't panic, though! It's just your friendly neighborhood dragon Drogon, landing to drop Daenerys off, probably so she can sic the Dothraki on the masters and handily win the battle. We'll have to wait to find out who comes out on top, but like, it's probably dragons. Just a guess.