Harry Harrison: Arm of the Law: Harry Harrison tells us what happens when a robot policeman is sent to a small town on Mars.

It was a big, coffin-shaped plywood box that looked like it weighed a ton. This brawny type just dumped it through the door of the police station and started away. I looked up from the blotter and shouted at the trucker's vanishing back.

'What the hell is that?'

'How should I know?' he said as he swung up into the cab. 'I just deliver, I don't X-ray 'em. It came on the morning rocket from earth is all I know.' He gunned the truck more than he had to and threw up a billowing cloud of red dust.

In the box the letter accompanying it said ' ... robots, correctly used will tend to prove invaluable in police work ... they want us to co-operate in a field test ... robot enclosed is the latest experimental model; valued at 120,000 credits.' So begins the saga of Ned, experimental robot policeman.