The Forget-Me-Not Killer

I know the truth. I know what you did...
Three retired men, Austin, Brian and Charles, each receive a greeting card with a bucolic painting of forget-me-not flowers on the front.
But the message inside is far from tranquil. It reminds them of the terrible deed they committed twenty years ago. All this time, they thought they'd got away with it, that no one knew. But someone, somewhere, does know.
And this someone is going to make them pay.
The three men have not spoken since that night. But now, in fear for their lives, they meet and discuss what's to be done. Between them, they convince themselves that it's an empty threat, the work of a sadistic joker.
Until Charles, on returning home, ends up dead; shot between the eyes, murdered on his own doorstep.
Detective Inspector Benedict Paige is assigned to investigate Charles' death.
But Austin and Brian know that unless Paige is up to the task, they'll be next...