On board a little tub of a boat, Homer and Joe, both in their fifties, sail a river up to waterfalls in Croatia. Until the recent death of their father, they did not know about each other's existence. Yet, they are half brothers. Sean, an enigmatic Irish adventurer, joins them. "Upstream" is a psychological adventure where these modest and solitary men will reveal some of their secrets, their hopes, perhaps, and where blood ties, whether they like it or not, will unite them.