In The Castle, aka Hillwood Mental Hospital, long-term patients are mysteriously killing themselves. Sean Rooney, trainee psychologist, forms a self-help patient group to investigate the mysterious deaths. The Castle has many secrets, some going back over a 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'. Rooney doesn't agree and after 'going undercover', believes there is more to these suicides.

All have a 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 historical, criminal, or supernatural. It takes a group of like-minded patients – a psychotic scientist, depressed philosopher, delusional vigilante, dope-head crime writer, autistic arsonist, wannabe detective, and a bipolar psychologist to find out who or what is killing patients at The Castle.

The Castle is the prequel to The Father, the first in the crime thriller series by critically acclaimed author Tom O. Keenan.