Mark Hamill says that the Mystery Inc. gang are "the guys who recently helped Batman rescue Alfred from the Joker", referencing the events of What a Night for a Dark Knight! (2019). When Daphne questions how he knew about that he says, "Oh, I have my sources", and then he starts to laugh maniacally like the Joker, which is the character he voiced in that episode.
One of three episodes to guest star Mark Hamill, but the only one in which he voices himself.