I'm dealing with Plop! the way I dealt with the mystery anthologies, so I am only stopping on issues in which Cain, Abel or Eve actually get integrated into one of the stories. The first time that happens is in Plop! 3 (Jan/Feb 74). Another creepy Wolverton cover, and the issue still has no ads.
Aragones does the framing sequences at the beach, as Abel asks Cain and Eve to explain exactly what a Plop is.
Michael Pellowski and Alfredo Alcala handle a story about goblins who keep trying to get published by Plop!, but keep receiving rejections slips. They get angry about this, and decide to go to Cain and show him just how scary they are, and by doing so frighten him into publishing their stories.
But before they ever meet Cain they run afoul of his gargoyle, Gregory, and take off screaming. Cain has no idea what is going on.
The conclusion of the framing sequence is on the interior back cover, as a huge wave plops Cain, Abel and Eve.