Harry Barnett thought he had left his military career behind, so he is startled when two figures from his past turn up on his doorstep after 50 years. An old friend has organised the reunion to end all reunions: a weekend in the Scottish castle where the ex-comrades took part in a psychological experiment many years before. They haven't seen each other since.

As they set off on their all-expenses-paid jaunt to Aberdeen, the old friends are in high spirits. But the cheerful atmosphere is quickly shattered by the apparent suicide of one of their party.

When a second death occurs, a sense of foreboding 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....