I will answer this question, but please don't let my plot summary and key events stop you from reading this brilliant novel as well. I hope that the key events that I give you will encourage you to read the book yourself, because it is an excellent read and I think you will enjoy it.
1) The novel begins as Michael and his family move into a dilapidated house. He and his family are greatly worried for the new baby that his mother has just given birth to, as her premature birth has placed her life at risk.
2) Michael finds a very peculair man in the garage who is at the point of death.
3) Michael starts to look after this man.
4) Michael hears a tale that says shoulder blades are what is left of angel wings, and Michael stops going to school and so becomes distant from his former friends.
5) Michael meets Mina, a neighbour who his homeschooled and deeply interested in nature and the poetry of william Blake.
6) Mina teaches Michael about her love of nature.
7) Michael introduces her to the strange man in his garage.
8) Mina and Michael move the man into an abandoned house because the garage is going to be destroyed.
9) They find out the old man has wings and that he actually appears to be beautiful and young.
10) The man introduces himself as "Skellig" to Michael and Mina and the owls in this new house feed him just as Michael and Mina feed him.
11) Skellig and the two children have a mystical, other-worldly experience where they can fly. Michael is mysteriously able to feel the heartbeat of his younger sister and believes that love is a force that can cure others.
12) Michael's younger sister's condition worsens and she must have heart surgery. His mother at hospital dreams that Skellig comes in and embraces the baby. Skellig disappears and the baby survives.
13) Michael continues being friends with Mina and with his former friends and develops his artistic talents.
14) Mina paints a picture of Skellig which is identified by Michael's mother.
15) At the end, everyone is happy and the baby sister is called Joy.