The boys are taking Olive on a picnic. It's April 1, and Bluto plays a series of "jokes" on Popeye, though of course they go beyond the bounds of acceptability, particularly once they get to the picnic grounds; Bluto puts gasoline on the fire he asks Popeye to light and swaps a beehive for the lemonade.
Bluto then launches a cruel joke against Olive and frames Popeye. The capper: he replaces Popeye's spinach with a joke can, and runs off with Olive for some canoeing.
But Popeye gets the last laugh with an inflatable sea monster.