Introduction <p>Mass casualty incidents (MCI) require a coordinated prehospital response from multiple healthcare teams. Undergraduate medical students rarely receive systematic MCI training, and some of the methods that have shown good results so far, full-scale simulations and extended reality (XR), tend to be expensive, logistically demanding, and not always reproducible across centres. The aim is to measure the change in knowledge scores from baseline to 6-month follow-up after a brief, low-cost gamified tabletop session, including the differential effect of sex and active role assignment, and to measure student experience with the game.</p> Methods <p>Quasi-experimental, single-group pretest/follow-up study with a knowledge assessment six months after the intervention; no immediate post-intervention knowledge test was administered. A total of 119 medical students from the Universidad de Oviedo took part in a 2-hour session using the MassCas tabletop game. Self-perception was measured with a 27-item self-efficacy questionnaire (Cronbach α = 0.926), covering methodology, knowledge acquisition, and skills acquisition (Likert 1–10). A 10-item knowledge test was administered before (<i>n</i> = 119) and six months after the session (<i>n</i> = 95), without any study material in between. Attrition was assessed by comparing measured baseline characteristics between respondents and non-respondents to the 6-month assessment. Effect sizes were calculated as r (Wilcoxon) and Cohen’s d. Sex and role assignment (active vs. passive) were explored as subgroup factors.</p> Results <p>Self-efficacy scores were high in all three dimensions: methodology (Median = 10; IQR = 2), knowledge acquisition (Median = 10; IQR = 1), and skills acquisition (Median = 9; IQR = 2). Knowledge scores were markedly higher at 6-month follow-up than at baseline: median pre-test 4 (IQR = 2) versus post-test 7 (IQR = 2); Wilcoxon <i>p</i> &lt; 0.001, <i>r</i> = 0.76, d = 1.19 (95% CI: 0.86–1.54). Among 63 email-matched pairs, 88.9% improved (<i>r</i> = 0.81, d = 1.44). Respondents and non-respondents to the post-test did not differ in pre-test score, sex, age, or role assignment (all <i>p</i> &gt; 0.10), with respect to these measured characteristics; differences in unmeasured characteristics cannot be excluded. Students with an active role rated methodology and skills dimensions somewhat higher; the largest difference was found for leadership (<i>p</i> = 0.007). Female students rated methodology slightly higher than males (<i>p</i> = 0.006). No statistically significant differences in 6-month knowledge scores were detected by role or sex. Prior MCI training hours were not related to self-efficacy scores. Qualitative analysis pointed to interactivity, visual format, and practical applicability as the main strengths, while excessive group size was the most frequently mentioned weakness.</p> Conclusions <p>After a single 2-hour session with the MassCas tabletop game, knowledge scores in undergraduate medical students were substantially higher at 6-month follow-up than at baseline, without any study material provided in between (d = 1.19), together with highly favorable self-reported perceptions of the methodology and of knowledge and skills acquisition. A low-cost, logistically simple methodology that does not require specialized equipment may be a feasible option where resources are limited. Active role assignment was associated with slightly higher skills perception, while no statistically significant difference in 6-month knowledge scores was detected between active and passive participants. These findings support further evaluation and implementation of gamified tabletop exercises in undergraduate medical curricula as an accessible and potentially scalable strategy for MCI preparedness.</p> Trial registration <p>ISRCTN56871134. Registered 23rd Jun, 2026 (Retrospectively registered).</p>