What happened before…
Owl Post and Phoenix Messages is not a story as such, but a collection of letters and newspaper articles.
It also is Part Two of the Wisdom and War Trilogy, which is HBP canon compliant, but AU to DH.
I strongly recommend that you read Part One, Madness of Minerva, first!
However, I know that there will always be some readers who just want to dive right into the story, so here's a complete summary of Madness of Minerva. But really – don't read the summary. Read the real fic.
Madness of Minerva starts with a missing moment in HBP and then goes on all through the summer holidays.
During the night before Albus Dumbledore's funeral, both Rubeus Hagrid and Minerva McGonagall hold vigil by his body. Hagrid eventually leaves the wake, knowing that Minerva and Albus were a couple and that she will want to say goodbye alone with him.
During her vigil, Minerva ponders how and if she will be able to go on without her lover and curses herself for ever trusting his murderer, Snape. Just before she wants to leave the room in the morning, she suddenly has the feeling that there is someone else in the room with her – but she cannot spot another (living) person in the chamber.
A short time later, Minerva meets Harry and his friends at Bill and Fleur's wedding. They start talking and eventually Harry confesses that he cannot take the pressure any more and does not know how to go on. He tells Remus Lupin, Tonks, Ginny, and Minerva about the Horcruxes and admits that he blames himself for Dumbledore's death.
The Order of the Phoenix decide that Harry ought to show them his memory of Dumbledore's death so they can discuss if he could have done anything to save the Headmaster. While they watch the scene, Minerva realises that Dumbledore did not beg Snape to spare his life, but to kill him.
From then on, Minerva tries to convince the Order that Snape is responsible for Dumbledore's death, but no murderer and certainly no Death Eater. She is torn between hating Snape for killing her lover and defending Snape because she knows that he only acted on Dumbledore's orders.
Regrettably, no one believes Minerva. She is by now so worn down with grief that the others believe her on the brink of insanity. In the end, Minerva herself begins to doubt her sanity because several times when she visits Dumbledore's grave, she senses a presence there. Poppy Pomfrey explains that these hallucinations are a normal reaction in her mourning state, but Minerva is deeply troubled.
Minerva leaves the Order because she knows that she is a burden to them if they think her mentally unstable. When a letter from Rufus Scrimgeour reaches her, telling her that Hogwarts will be closed, Minerva totally breaks down.
She spends the night at Dumbledore's grave again, growing increasingly angry how he could have left her like this, without any explanation or information how to go on. In her rage and sorrow, she collapses by the tomb and encounters the ghostly presence again.
In the end, Minerva realises that it was no ghost she has met all those times, but Snape, who is driven to the grave by his own feeling of guilt. She confronts him, and after a very emotional discussion they realise that their only chance to help Harry, Ron, and Hermione is to work together.
So now... go for Part Two, Owl Post and Phoenix Messages await you!