A spoof on the NBC hit sketch comedy series Saturday Night Live, Castle’s latest episode, “Dead From New York,” embarks on a murder mystery involving the death of the producer Sid Ross (actor uncredited) of “SNT,” also known as “Saturday Night Tonight.” The list of potential suspects varies, including the current host, a suave and dashing actor named Danny Valentine (guest star Gregory Harrison). And like the real show, there’s always a musical guest and this week’s guest is Carly Rae Jepsen (herself).