What followed was so simple. So straightforward.

Alice headed for the pub and Rosa turned and walked in the opposite direction. When she looked back and saw that Alice had disappeared inside, she stopped and took stock of the situation.

The police car was still there and there was only one occupant – a woman officer in the driver’s seat. She was perhaps taking time out or waiting for a colleague to appear. But from Rosa’s point of view she was the proverbial sitting duck.

Rosa looked around, noting that there were no CCTV cameras focused on the immediate area. The few people who were around were simply passing by and none of them seemed to even notice the police car that was parked about ten metres back from the riverside walkway with only its side lights on.

Rosa dipped her hand into her bag as she strode purposefully towards the car. When she drew level with it she stopped and turned and was glad to see that the walkway was now clear, although she could hear voices.

She had approached targets in exactly this way before. The secret of success was to act natural but at the same time move with the speed of a striking cobra.

She peered through the passenger-side window and saw that the cop was looking at her cell phone. When Rosa tapped on the glass the woman turned sharply and a frown creased her brow.

She appeared to be in her thirties and was rather pretty.

Rosa didn’t try to open the door because she knew instinctively that there was no need. The cop did not perceive her to be a threat and proceeded to lower the window.

And that proved to be the last mistake she would ever make.

‘What’s the problem, miss?’ the cop asked her.

Rosa glanced quickly around and saw there was nobody watching. Then she whipped the pistol from her bag, thrust it through the open window and fired three shots into the woman’s body.

The shock registered on the cop’s face but only for a split second. And she didn’t make a sound other than to expel a lungful of air.

Then she slumped sideways across the passenger seat from where she couldn’t be seen from a distance.

Rosa swiftly withdrew the gun and slipped it back into her bag. She stepped back and checked again that she wasn’t being watched. The walkway and the street were both still empty.

As she walked away from the car she knew that it could not have gone any better. It had been a slick, clean hit. Easy and effortless, which was usually the case with random executions. There was no need to plan them. No need to agonise over the approach or the exit route. All you had to do was seize an opportunity and that was exactly what she had done.