Kinderpalliatieve zorg is iets waar je nooit over nadenkt… tot het opeens deel van je leven wordt. Het is zorg die er is wanneer niets je kind meer beter kan maken. Wanneer er nog wel medicatie is, ma...

Background: Advance care planning (ACP) is an ongoing dialogical process. Despite growing policy-level recognition, ACP engagement remains limited in Japan, and many individuals remain unfamiliar with the concept. Beyond knowledge deficits, insufficient psychological readiness may also impede engagement. Evidence on fully online narrative- and game-based approaches to supporting such readiness remains limited. Objective: This study aimed to explore the #feasibility and acceptability of a fully web-based ACP educational intervention integrating an explanatory video and an interactive game and to evaluate postintervention cognitive, attitudinal, and psychological responses. Methods: This exploratory cross-sectional online study was conducted in Japan between July and August 2025. The final analytic sample comprised 158 adults, including 46 with health care or long-term care experience and 112 without. Participants watched an animated explainer video, played an interactive narrative game, and completed a postintervention questionnaire online. #feasibility and acceptability were assessed based on participation and response distributions. Responses to 12 questionnaire items (Q7-Q18) were rated on a 5-point Likert scale. Between-group comparisons were conducted using 2-tailed Welch tests (α=.05). Open-ended responses were organized into broad descriptive themes. Results: Participants' mean age was 44.4 (SD 11.5) years, and 36.1% (57/158) were female. Postintervention responses showed generally favorable engagement. Mean scores exceeded the scale midpoint (3.0) for most items, including improved understanding of ACP (Q8: mean 4.39, SD 0.75), perceived importance of ACP (Q13: mean 4.42, SD 0.68), and personal relevance (Q15: mean 4.42, SD 0.83). Responses showed greater variability for imagining a concrete ACP discussion (Q17: mean 3.72, SD 1.02) and perceived psychological barriers (Q18: mean 3.49, SD 1.17). Significant between-group differences were observed for Q7 (mean difference −2.49, 95% CI −3.01 to −1.97;