I used to be a fan of the TV show Dexter, I say “used to be”, because until the last season it was a thoroughly enjoyable (and) guilty pleasure of mine. However, season 8 ruined it for me. The twists and turns in the plot were nothing short of nonsensical (I could give examples; Deborah's sudden XXX but I might spoil it for those who have yet to watch this disappointing season. A word of advice, don't.)
The grand finale of the show struck me speechless, I would describe it as being a cop-out ending, an ending which had no premise, a desperate attempt by the screenwriters to give a final dramatic twist to the story, but which fell flat on its face. I believe that the writers were trying to attempt a sort of poetic justice, an ironic ending considering Dexter's character at the beginning of the TV show. But it just didn't work.
Is there a word or expression which describes a sudden and inexplicable "cop-out ending", wherein the reader of a novel or TV viewer feels cheated in some way.
Can I say: “a cop-out ending”? Is it idiomatic?