Following a forced landing on Harmonia in search of spare parts, Tom Mishkin is provided with a SPER robot for his personal protection. Trouble is, the robot was programmed for Darius IV and shipped to the wrong planet in one of those administrative foul-ups so common to the best of all possible worlds. Despite a glib line of street-wise patter, Mishkin's robotic companion has no more idea of what's going on than his reluctant master. Which is a shame - as the intrepid pair are assaulted by a succession of dangerous hallucinations in a number of parallel universes. Is this a Kantian universe or is David Hume to blame? Nothing is certain except the infinity of Options in Robert Sheckley's hilariously unstable cosmos.