Secrets of Haunted House begins with the April/May 1975 issue.
The book has four hosts: Cain, Abel, Eve, and Destiny. Aside from the first three appearing in Plop!, each of those was also hosting a different book in DC's mystery line, House of Mystery, House of Secrets, and Weird Mystery Tales, respectively. Destiny had debuted in Weird Mystery Tales, but been slowly shoved out by Eve. Steve Skeates and Ricardo Villamonte provide the framing sequence, in which Destiny comes off as far more dire and serious then the other three.
They all wind up squabbling over the Book of Destiny, a fight which results in Abel falling off a cliff. That is possibly due to the fact that Abel does not narrate one of the stories in the issue.
Destiny gets to relate the first story, which deals with two ambulance drivers. I have a suspicion that this may have been a leftover, intended for Weird Mystery Tales, but shelved when Eve took sole hosting duties there. That would make Secrets of Haunted House sort of a catch all for leftover stories that didn't make it into any of the other books.
Michael Pellowski and Robert Kanigher script the Ernie Chan story, but why it took two writers is beyond me. It's a simple and obvious tale. The ambulance drivers always need to speed around a deadly curve in the road, a place many cars went over the cliff. The drivers play a game, recording whether the people in the accidents lived or died. And then, at the end of the story, the game ends as well when they die.
Cain hosts the second story in the issue, by Jack Oleck and Alex Nino. Some really lovely art on this tale, about a couple who find a strange aquatic beast in a nearby lake. They bring it to their home, only to find the creature is able to telepathically control them.
The beast grows larger and more powerful, and is able to kill their mailman without even making contact with him, due to his mental powers. It seems all but unkillable, and it takes off at the end of the story, diving into the sea to begin it's conquest of the world. Except it was a freshwater animal, and dies in salt water. D'oh!
Eve doesn't get a full story to narrate, but instead hosts a page of Sergio Aragones cartoons.
The framing sequence closes out the book, revealing that Abel did not fall to his death, he's just been hanging from a tree root.