A theory-led narrative approach was used to unpack the complexities of the factors that enable successful client adherence following one-to-one health interventions. Understanding this could prepare the provider to anticipate different adherence behaviours by clients, allowing them to tailor their interventions to increase the likelihood of adherence. The review was done in two stages. A theoretical formulation was proposed to explore factors which influence the effectiveness of one-to-one interventions to result in client adherence. The second stage tested this theory using a narrative synthesis approach. Eleven studies across the health care arena were included in the synthesis and explored the interplay between client attachment style, client-provider interaction and client adherence with health interventions. It emerged that adherence results substantially because of the relationship that the client has with the provider, which is amplified or diminished by the client's own attachment style. This occurs because the client's attachment style shapes how they perceive and behave in relationships with the health-care providers, who become the 'secure base' from which the client accepts, assimilates and adheres with the recommended health intervention. The pathway from one-to-one interventions to adherence is explained using moderated mediation and mediated moderation models.