The envelope is unremarkable. There is no return address. It contains a single, folded, sheet of white paper. The envelope drops through the mail slot like any other piece of post. But for the nine complete strangers who receive it - each of them recognising just one name, their own, on the enclosed list - it will be the most life altering letter they ever receive. It could also be the last, as one by one, they start to meet their end.