The digital therapy intervention for smoking cessation, ‘Happy Ending’, has been shown to be efficacious in two previous randomised controlled trials. The aim of the current article is to disentangle the rationale of the intervention and describe its development. For this purpose, Intervention Mapping is used as a descriptive tool. The intervention is fully automated and delivered by means of the Internet and mobile phones. It is based on self-regulation theory, social cognitive theory, cognitive–behaviour therapy, motivational interviewing and relapse prevention. The ordering of the content is based on a reasoned chronology, modelled according to psychological processes that people experience at certain time points in a process of therapy-supported self-regulation. The design of the intervention is innovative in that it combines four media channels (SMS, IVR, e-mail, and web), and in the combination of just-in-time therapy and a tunnelling strategy based on the natural chronology of quitting. The two forms of just-in-time therapy are a craving helpline (mainly targeting negative affect), and the provision of relapse therapy based on a daily assessment of the target behaviour. The present article meets the recent calls for more thorough descriptions of interventions, and may inform systematic reviews and the development of interventions.