This was my absolute favorite episode when I was a kid.  I loved episodes that appeared to be supernatural but then turned out not to be when a rational explanation appeared.  There's still a plot hole, though, a loose end in the form of Del Delahanty.  He told Redmond that he lied when Andrea called...except he did not.  He had to have been in on the whole thing, and with enough pressure, would certainly have squealed.  They didn't need to trick Andrea into giving herself away.  Mystery; imagined supernatural elements.