"Plain old passion" means "what everyone calls passion".

Its a rather common expression, used to emphasize something that might appear to be ordinary, to emphasize what most people may have overlooked.

Typical phrases:
"Plain old vanilla ice cream" (ordinary everyday vanilla ice cream)
"plain old love" as in "he was snared by plain old love" (he was trapped by good old-fashioned love)
"plain-old plain old" as in "he gave me the plain-old plain old ("he told me all the things you would expect to hear him say" *OR* "he told me the unfettered truth")