I think that the plot will be rather complicated, so here goes on what I think.
We'll start out at the Order's new headquarters. The Order will be in a meeting, deciding what to do post-Dumbledore's death. In the end they choose McGonagall becuase of the fact that Hogwarts has been closed by the Governors until Lord Voldemort is defeated. Then it cuts to Harry who is at the Weasleys right before the wedding. He will be asked to accompany Ginny down the aisle and will realize that the tiara which Ginny will be carrying to Fleur belonged to Rowena Ravenclaw, becuase it is engraved on the inside. There will be a small confrontation over the tiara and Harry will destroy it on the spot, causing an explosion. The explosion injures the Weasley twins and kills Shacklebolt.
After sorting out the issues of the aftermath wedding, Harry, Hermione and Ron return to Privet Drive so Harry can stay there for the rest of July. During his stay there Harry finds out what it was the Dudley saw when the dementors attacked him. He saw himself at his school, being beaten for being a bully. We get to see a sorrowful and crying Dudley, just like Draco in book 6.
Harry leaves Privet Drive and wanders around a little bit, visiting Godric's Hollow first, where he finds nothing. He then heads to the new headquarters of the Order of the Phoenix. He stays there until early fall, keeping up to date on Voldemort and the Death Eaters. In Mid-September he leaves with Lupin to once again travel to Godric's Hollow, Lupin wanting to see it for himself again. While there, Lupin tells Harry what the Order had figured out about the special circumstance of Lily being given the chance to live. It was becuase it was requested of a Death Eater of Voldemort's, one that Voldemort thought most highly of. Harry believes it to be Snape, but Lupin is skeptical, thinking Snape to have been good then.
The two of them then return back to headquarters. There, he encounters the broken pieces of the mirror Sirius gave him. He repairs it and tries to talk to Sirius. This time, a blurry face appears and Harry hears his Godfather's voice. Eventually, it clears and he is able to talk to Sirius. Sirius then hands the mirror to James and Lily respectively. Harry recounts everything that has happened and mentions the locket. At this, Sirius reacts. He mentions that in Harry's 5th year, Snape arrived at Grimmauld Place and entrusted Sirius with a small box. It was odd, but Sirius accepted it. He looked inside and saw a heavy golden locket, but he forgot about as Sirius did not believe that it was dangerous. Still, it was still probably at Grimmauld Place. Harry then travels to the Black residence to destroy the locket. There, he finds the locket but he is not alone. Snape is also at Grimmauld Place and disarms Harry. Snape explains to Harry why Dumbledore trusted him, not saying who's side he was on. That remains a mystery. Then, he asks Harry why he is at Grimmauld Place, but Harry refuses to answer. That is when Hermione and Ron arrive, threatening Snape. In the end Harry lets Snape go to bring a message back to the Dark Lord that Harry will kill him in the end. Harry summons Kreacher and together they find the locket, destroying it, which causes the house to start to crumble, as if it were the cave. Hermione, Ron and Harry barely survive, but Kreacher is crushed, making the last of the Blacks, elf and human, gone from the Earth.
Afterwards, the trio journies to the Ministry of Magic and makes a visit to the Department of Mysteries, where they study the veil. Harry contacts Sirius again, asking him how to get him out of there, as well as possibly his parents. Sirius tells him there is no way and explains what the veil is, to Harry and the confounded Unspeakables present. The veil is a device that disintergrates the body as it falls through, thereby killing you. There is no way to come back. After leaving, Harry decides to start tracking the third horcrux he must destroy, but is joined by Neville and Ginny who are determined to help Harry in whataver it may be. Harry takes them into his consequence and together the continue the search. Voldemort send Nagini after Harry, to kill his friends and make Harry even more desperate. Nagini is not successful as she only is able to bite Neville and not fatally. Neville is taken to St. Mungos after Harry kills Nagini, using the same spell Snape used on the snake during the dueling club.
As Harry and the gang leave St. Mungo's will Neville after he recovers, Harry is approached by a man. He recognizes him as the lead singer in a band, the same man that the Quibbler had reported was Sirius two years back; Stubby Boardman. Stubby takes Harry and the rest to the Leaky Cauldron, where he admits to Harry that he is the 'long-dead' Regulas Black, brother of Sirius. He admits to having been part of a small secret group with-in the Death Eaters that had been slowly making Voldemort easier to destroy. He tells Harry that with the help of Kreacher he stole the locket from the cave, going on some evidence from an old acquaintance of the young Tom Riddle, Amy Benson. He gave the locket to another member of the group, to be destroyed. That person was Severus Snape, who admitted to 'destroying' the locket, before tearing the group down from the inside. Many were killed, but Regulas escaped, ashamed of being unsuccessful. Regulas also comments on Harry's eyes, saying that the one that loved Lily will probably never kill Harry, becuase of his eyes. He still does not say who it is, believing Harry needs to find out for himself. He agrees to help Harry find the last horcrux though, copmleting his goal.
After a while together, Harry and Ginny decide that their destinies are now one and they get back together. (Enough said there.

) Then, the search for the last horcrux begins, going on some information that Regulas has for Harry. The group travels to the Riddle house, where Regulas is sure that either the horcrux is or the next clue in a long line of clues. There Harry encounters Voldemort. Behind Voldemort is Pettigrew and a couple other Death Eaters. Voldemort casts the Cruciatus Curse on Harry, but is shocked when Harry AND himself both feel the effects of the curse. Harry realizes it is becuase they share the same blood and that the final battle will not be ended with magic. Voldemort tells Bellatrix Lestrange to kill Harry, not realizing that it will kill him as well. But the spell never reaches Harry. Pettigrew, realizing who is right and wrong, dives in front of the curse, taking the full blow and crumpling to the floor. This gives the rest of Harry's group time to cause a commotion, bringing down the roof and giving them a chance to escape. Regulas leads them down the graveyard where there is a small battle between Ron, Hermione, Neville and Ginny against Bellatrix Lestrange, Draco Malfoy and Fenrir Greyback, while Harry and Regulas dig furiously at Tom Riddle Sr.'s grave site for the cup, which they have figured out resides there, due to an inscription on the back of his tombstone. Neville avenges his parents, stunning Bellatrix and sending her back into a tombstone. Ginny deals with Fenrir with her famous Bat-Bogey curse, knocking him out near Riddle's Gravestone. Hermione and Ron team up and corner Draco, but keeping him alive. Harry and Regulas are unsuccessful with the locket, so they blast the gravestone away and it lands on Greyback, crushing his body. They recover the cup and Regulas uses the proper spell to destroy it, making it dissolve. Voldemort is now mortal, but they need to recover. Taking Draco, they return to headquarters. There Harry demands of Regulas who the person who loved his mother was, as they need an ally. He says it is Snape.
Draco becomes a double agent and they let him go. Draco returns to Voldemort and lets Harry and the Order know of his exact location. He has moved from the Riddle House. He has set up his new headquarters at Azkaban, the dementors having revolted. The order sets out for Azkaban and a great battle insues. Regulas is knocked out by Snape, before losing his soul to one of the dementors. Harry and Snape have an encounter before Harry encounters Voldemort. Harry asked him why he killed Dumbledore and Snape answers that is was necesary. Dumbledore only stood in the way of Voldemort. At this moment, Fawkes appears carrying Godric Gryffindor's sword. He drops it to Harry before attacking Snape. When Fawkes is done with Snape, he is bloody and dead. Fawkes perches on Harry's shoulder and together they walk through Azkaban, trying to locate Voldemort. They eventually find him in a cell with Lucius Malfoy and Draco. When Harry attacks Voldemort blasts the sword out of his hands. Voldemort tells Harry that he is going to kill him, even though it will kill Voldemort as well. He tells Harry he has horcruxes and that a loyal Death Eater will bring him back again. Right before he kills Harry, Draco picks up the sword of Gryffindor and brings it down on Voldemort's head, ending the reign of the Dark Lord.