Castiel needs to do a little time traveling to protect a Campbell ancestor from someone determined to stop the Winchesters from ever being born. He moonlights as a navigator on a World War II bombing run that also happens to contain one Captain Jack Harkness, on a mission of his own, impersonating an American soldier.
Both of them realize at some point that the other is a covert operative, but for the sake of their own missions, determine the other isn’t a threat and let it go at that.
The plane goes down. Cas resurrects Great Grandpa Campbell and Jack is, well, Jack, but now the three of them are behind enemy lines and have to get to safety. At some point the entity trying to off Great Grandpa Campbell rears its ugly head. Turns out it’s the thing Jack was trying to take out, too.
They part ways after getting to an allied base – Cas determined to tag along after Great Grandpa Campbell for a little while longer, just in case. At some point this picture gets snapped, Jack autographs it, and Cas has been carrying it around with him ever since.
Even if Jack misspelled his name.