I remember watching this show with my brother and parents when it was on in the 80's, and I remember the previews for an episode where they said Magnum must solve the riddle or die. Now I'm thinking riddles are kind of fun so I'll make it easy by saying the riddle is ,"Why'd the chicken cross the road?" I'm sure Tom Selleck's character would've loved it if the riddle was that easy.
Take care and may God bless.