All of the Old Gods, from all the world's myths, are real beings who once walked among humanity, protecting, guiding, and punishing as They saw fit. They left, long ago, left us to grow and learn as we would, and now They are coming back, to see if we have done as well as They expected. However, They can no longer maintain bodies of Their own, and so They have to possess a human (the one human in the whole world who is a perfect match for one of Them), and share the human's body. In return, the human gets certain magical powers, and vast amounts of knowledge.
The Gods, as They learn about humans and how we have developed, break roughly into two camps: the pro-human faction who think we've done quite well without Them, and want to go back to guiding and advising us, or even to start collaborating with us; and the anti-human faction, who think we have abused Their gifts, and want to take back the gifts, or free will, or even our lives. Our Heroes must rally the pro-human groups into an effective force, fend off the anti-human groups, and contend with monsters and creatures, all while trying to maintain their grades and learn how to be adults. Good thing they've got each other!