Dewi's Fish is the name given to a literary device that is related to the narrative technique of 'Chekov's gun'. Chekov's Gun says that if a gun is shown in the story then it must be fired at some point. Dewi's Fish says that if a fish is thrown at someone on page 300, the fish's existence must have been flagged to the reader on page 100. In other words, you should never surprise the reader. Unfortunately, a lot of TV (especially sci-fi) does not obey the principle of Dewi's Fish in writing.