Background: Rates of compulsory (also known as involuntary) detention under mental health legislation have been rising over several decades in countries including England. Avoiding such detentions should be a high priority given their potentially traumatic nature and departure from usual ethical principles of consent and collaboration. Those who have been detained previously are at high risk of being detained again, and thus a priority group for preventive interventions. In a very sparse literature, interventions based on crisis planning emerge as having more supporting evidence than other approaches to preventing compulsory detention. Method: We have adapted and manualised an intervention previously trialled in Zürich Switzerland, aimed at reducing future compulsory detentions among people being discharged following a psychiatric admission that has included a period of compulsory detention. A co-production group including people with relevant lived and clinical experience has co-designed the adaptations to the intervention, drawing on evidence on crisis planning and self-management and on qualitative interviews with service users and clinicians. We will conduct a randomised controlled feasibility trial of the intervention, randomising 80 participants to either the intervention in addition to usual care, or usual care only. Feasibility and acceptability of the intervention and trial procedures will be assessed through process evaluation (including rates of randomisation, recruitment, and retention) and qualitative interviews. We will also assess and report on planned trial outcomes. The planned primary outcome for a full trial is repeat compulsory detention within one year of randomisation, and secondary outcomes include compulsory detention within 2 years, and symptoms, service satisfaction, self-rated recovery, self-management confidence, and service engagement. A health economic evaluation is also included. Discussion: This feasibility study, and any subsequent full trial, will add to a currently limited literature on interventions to prevent involuntary detention, a goal valued highly by service users, carers, clinicians, and policymakers. There are significant potential impediments to recruiting and retaining this group, whose experiences of mental health care have often been negative and traumatising, and who are at high risk of disengagement. Trial registration: ISRCTN, ISRCTN11627644. Registered 25th May 2022, https://www.isrctn.com/ISRCTN11627644.