Play It Again, Jack

Captain Jack Harkness is in the process of re-forming Torchwood, when the Leviathan arrive, looking for him. He's rescued short of catastrophe by Castiel: pushy, arrogant, humorless, and more buttoned up than the Royal Guard on parade day. Thus Jack finds himself on a mission to defeat a quasi-biblical, ferociously hungry race, change history and save the world. Again.