Attachment and exploration are closely related behavioral systems. One way of activating both systems in children is the use of narrative story stems—a doll-play procedure in which the adult gives the child the beginning of a story (the stem) and asks them to finish it by “telling and showing me what happens next.” This paper presents the Child Attachment and Play Assessment (CAPA)—a new system for analyzing story stems with preschool (age 3–6 years) and school-age children (7–11 years). The CAPA provides a reliable means of assessing children’s attachment strategies using Crittenden’s Dynamic Maturational Model (DMM) of attachment together with play-based dimensions that assess mentalizing (i.e., the ability to think about ourselves and others in terms of intentional mental states together with the awareness that the contents of our mind may be different to that of people). A preliminary validation study was conducted establishing concurrent validity with the DMM-CARE-Index and a Child Attachment Interview for preschool and school-age children, respectively. The discrimination between risk and community children was excellent and there was good inter-rater reliability.