As Liverpool's Special Murder Investigation Squad prepares for Christmas, Detective Inspector Andy Ross receives a phone call. Someone has murdered Santa Claus (well, a department store Santa). As the officer on call for the night, Ross attends with his sergeant Izzie Drake, and begins one of the strangest investigations of his career. With the main suspects being Santa Claus himself, in cahoots with an elf, Ross needs luck - and lots of it - as he seeks to solve the mystery of A Merry Mersey Christmas.