As for a brief rundown of the plot: Harry is back in the public eye, and things are getting pretty dang crazy. Cornelius Fudge was sacked as Minister of Magic because of his lack of action at the warnings of Voldemort's return (he claimed Harry was making up stories and it wasn't true). Now, Rufus Scrimgeour is the new Minister, and Voldemort and his Death Eaters have no problem making their presence known, as it's already out in the open. Destruction and chaos have gotten of hold of not only Wizarding England, but Muggle England as well. Harry is taken on a trip with Dumbledore shortly before school begins to find a Horace Slughorn, former potions master. Horace is enticed by Harry, being quite fond of getting in with well-known, high-ranking people, and agrees to return to Hogwarts... to once again be potions master. This means that Snape gets to finally get his dream position of professor of Defense Against the Dark Arts. Snape's had a busy summer as well, making an unbreakable vow with Narcissa Malfoy, Draco Malfoy's mother, that he would do everything he could to help Draco with whatever task Voldemort had assigned him.
Harry returns to school and finds himself able to continue with potions due to Professor Slughorn's less strict requirements. He borrows a used book, which has lots of notes in the margin that help Harry not only do much better at Potions than usual, but actually excel in the class. The year seems pretty typical, except for one extra "class" Harry has: he meets with Dumbledore and, through his Pensieve, and using memories that he's collected, they're able to see some of Tom Riddle's history. Harry is able to get an untampered memory from Professor Slughorn which solidifies Dumbledore's guess that Voldemort has been using Horcruxes, a very dark magic where one rips their soul, and places it in an object, allowing them to survive even after death. They have enough evidence to know several key points: He made 7, they've destroyed 2, and Tom would use artifacts from the founders of Hogwarts. Dumbledore was able to locate one, and they take off to get it. The protection that Dumbledore has to go through to get it leaves him very weakened, and when they return to Hogwarts and find it overrun with Death Eaters, things don't look good. Upon landing in one of the towers, Dumbledore quickly stuns Harry, making it impossible for him to move. Out of sight, Harry can only listen and watch as Draco comes to try and kill Dumbledore. When he realizes he can't do it, Snape suddenly barges in and kills him. Harry is now free, because a spell can't continue once the caster is dead. He chases after Snape and tries to get revenge, but to no avail. Harry is now without Dumbledore, his mentor and greatest form of protection, and he now has to go out and accomplish what Dumbledore had started, with little knowledge of what to do, little idea of where to go, and little preparation for what he was going to face.
Harry had relied on Dumbledore these past 5 years for just about everything. He felt comfortable and safe with Dumbledore around, and knew that nothing bad could happen to him. He learned a lot from him, and discovered things about himself, his family, his past, present, and future. Dumbledore was his guiding light; but after that night, Harry was alone. Harry never considered the event of Dumbledore's death, or what that would mean. He always assumed he'd be there to help him. But the end came for him, and Harry was left to face this great adventure without him. Now, he could no longer rely on Dumbledore, he had to do it all on his own.
As we grow up, we lean a lot on our parents and leaders for lots of things. We gain opinions and tastes, likes and dislikes, morals and beliefs from the people who raise us. This is common, it's hard for a young child to make a well formed view of a serious issue. So, they simply take from their parents or their teachers of their church leaders those things. The same is true of a testimony. We often see small children go and bare their testimony: "I know the church is true, I love my mom and dad, I know Heavenly Father loves me, name of Jesus Christ amen." Classic. But, their testimony is simply an appendage of their parents'. They know the church is true cause mom and dad do, and they told them so. Because their Bishop does, and he told them so. Because their Primary teachers do, and they told them so. They don't truly have their own testimony. They don't have a rock solid foundation. They can't, they're simply too young. It's when we grow up and start to learn things for ourselves that we can come to certain important conclusions. At some point everyone, everyone, must decide for themselves if this church is true. Everyone from you to President Monson. We need to read the Book of Mormon and pray, to truly and sincerely ask if it is true, and the Lord will tell us. It's only when He tells us that we will truly know that it is true. We're just like Harry: eventually, we're expected to do things on our own. We can't just ride on the coattails of others, we need to be able to stand on our own. Just as Harry took on Voldemort and his army, even without Dumbledore, we can all take on Satan and his army, even on our own. As we develop and strengthen our individual testimonies, and nourish the seed (Alma 32), we'll be able to face whatever opposition comes our way.