58 parts Complete Angels, demons, Lilith and the 66 seals.
Ever since the intervention of rescuing the eldest Winchester from Hell and bringing him back to life, the angels explain that they've arrived on Earth for the first time in two decades in order to prevent the demons from freeing the Fallen Angel, Lucifer, from Hell, thus causing the Apocalypse.
With the demons being led by the Winchester brothers' enemy and the eldest's murderer, Lilith, it becomes increasingly clear that something is wrong with Heaven and that the Angels have their own agendas.
The brothers, with the help of their childhood friends, the Hale sisters, must prevent the 66 seals from being broken to free Lucifer, all while tension grows over the younger brother's growing demonic powers and the distrust over his returning demonic ally.
Sounds easy enough, right?
° Book One. Based on Supernatural: Season 4 °