For a group of ex-comrades, it is to be the reunion to end all reunions: a weekend in the Scottish castle where they were guinea pigs in a psychological experiment many years before. They haven't seen each other since. But the convivial atmosphere on the journey north is quickly shattered by the apparent suicide of one of their party. When a second death occurs, a sense of forboding descends on the group. It appears that the past is coming back to haunt them, a past that none of them have ever spoken about. Their recollections are all frighteningly different. So what really happened? Then when one of them uncovers an extraordinary secret, he becomes convinced that they will never leave the castle alive...