Whew, we're in luck: Harry, Hermione, and Ron seem to have a plan. They're going to break into the Ministry of Magic and steal it back.
Simple, right? Wait, no—it sounds a) really dangerous and b) basically impossible.
The plan involves knocking out a few Ministry employees, stealing their hair, and then making/using Polyjuice Potion to turn themselves into these folks so they can get in the Ministry undetected.
The first steps go off according to plan, though, so Harry, Ron, and Hermione end up in disguise and ready to enter the Ministry.
So far, so good—they're in. But they have to interact with people as the folks they're impersonating, which is kinda difficult…since they have no idea who these people are, other than the fact they work at the Ministry.
Eventually, they all have to split up, and Harry (as "Albert Runcorn") goes exploring, and Hermione actually gets pulled away to do something with Umbridge.
Harry manages to find Umbridge's office—and sees Mad Eye's eye in the door.
He then plants a diversion and gets into the office to look around. He doesn't see the locket anywhere. However, he finds files on his friends and Dumbledore.
Harry and Ron then find each other again, and they go to find Hermione, who's in the basement.
When they get there, they find that Umbridge is actually wearing the Horcrux. That's both lucky and unlucky. Again, how are they going to get it?
We should probably mention what Hermione and Umbridge are doing in the basement: They're actually in a courtroom, and a woman named Mary Cattermole, the wife of the dude Ron is impersonating, is on trial, apparently for maybe not having enough wizard blood or something like that.
Harry manages to attack Umbridge and get the Horcrux from Umbridge. The kids—with Mary in tow—escape from the courtroom.
By this point, the Polyjuice Potion has worn off, and it doesn't take long for people to realize the kids are intruders. They rush to get out of the Ministry.
They make it out, but one of the men pursuing them was holding on to them when they tried to leave…
When they land, something brutal has happened to Ron. It seems that when Hermione was trying to Disapparate and that dude was holding on to them, something called Splinching happened. He's got some really nasty looking wounds on his body.
And in other bad news, now they can't go back to Grimmauld place. You see, Hermione had tried to Apparate there first, before she realized that she had brought an enemy along, and that means the dude now has access to Grimmauld.
Time for some good news, though: Hermione can heal Ron's wounds pretty quickly with something called Dittany. They're all still pretty shaken up, but at least Ron will live.