Why have Thames Valley Police banned film crews from TV detective series Lewis from filming at St Aldate's police station in Oxford?
It's the kind of mystery that would have completely baffled Inspector Morse, leaving him frustrated after a number of false trails led to dead ends.
Such a puzzle would be solved only after his faithful assistant Sgt Lewis, played by Kevin Whately, stumbled across the solution, prompting Morse to choke on his pint and exclaim "You've done it, Lewis!"
In the spin-off series Lewis, which re-starts tomorrow night, Morse's former assistant works for the Oxfordshire Constabulary, which was abolished when it amalgamated with neighbouring forces to form Thames Valley Police in 1968.
Toby Shergold, a spokesman for Thames Valley Police, said: "A decision was taken some time ago that the Thames Valley Police name should not be used in any fictional police series because we are not a fictional force.
"If TV producers were to use our name, we would want some input into the programme to ensure the force was correctly portrayed and that is not practical for either side."
Mr Shergold added that the logo was copyright-protected and couldn't be used without permission.
Dr Antony Richards, who runs the Inspector Morse Society, said he was surprised by Thames Valley Police's stance as some officers, including Mr Trotman, were honorary members of the society.
But he added: "There was a precedent in 1994 when Thames Valley Police agreed that we could use the old Oxfordshire Constabulary logo on our society ties.
"Then they changed their minds and we had to destroy about 100 of the ties."
Colin Dexter, who wrote the Morse series which the TV dramas were based on, has a cameo role in tomorrow night's drama.
He said: "Even if there has been some directive about not filming at St Aldate's, there has never been any ill-will between the TV companies and the police."