Sean Rooney, psychologist and erstwhile psychosleuth, spent nine formative months as a trainee in Hillwood Mental Hospital, aka, the Castle. In The Castle, long-term patients are mysteriously and suspiciously killing themselves. Rooney forms a self-help patient group to investigate the mysterious deaths.

The Castle has many secrets, some going back over one hundred years. Rooney has a particular reason for choosing the Castle as his placement, posing a question: Is he there to meet his own needs or that of the patients? The hospital management team consider suicide in large mental hospitals as coming 'with the turf', but Rooney doesn't agree and after 'going overcover', believes there is more to these suicides. All have one common feature: after many years in hospital, these patients were all considered for 'care in the community'.

The Castle doesn't give up its secrets easily, whether they be historical, criminal, or supernatural. It takes a group of likeminded patients to discover what is really going on in the Castle. So, what do a psychotic scientist, a depressed philosopher, a delusional vigilante, a dope-head crime writer, an autistic arsonist, a wannabe detective, and a bipolar psychologist, all have in common? They all want to find out who, or what, is killing the patients of the Castle.