This study broadens the current understanding of the role of planning by focusing on the interplay between individual and dyadic planning (i.e. making plans about the target person's behaviour together with a partner). Self-report data from N=141 prostatectomy-patients and their partners were assessed at three times within 1 year post-surgery. Direct and indirect effects of dyadic and individual planning on patients' pelvic-floor exercise (PFE) were tested. Proposed mediators were social support, social control, and action control. Cross-sectionally, the dyadic planning-PFE relationship was mediated by patients' received support and partners' provided social control. Longitudinally, mediators of dyadic planning were partners' provided social control and support. Effects of individual planning on PFE were mediated by action control at baseline only. Also, at lower levels of individual planning, patients' dyadic planning was more strongly associated with receipt of social control. Results underscore the importance of social factors in the planning process and its mechanisms in health-behaviour change.