Abstract Most symptomatic Latina women do not receive a diagnosis or initiate mental health (MH) treatment (Tx) when warranted. Stigma, cultural concerns, distrust of professionals, lack of insurance, and not knowing where to get help are barriers so many suffer from untreated depression and/or anxiety. Due to high social media and YouTube use and high demand for story-based media among Latinas, however, our multidisciplinary team created and tested a choice-driven, Hollywood-quality web-based app featuring transmedia storytelling videos called Catalina: Confronting My Emotions/ Enfrentando Mis Emociones (in English and Spanish) to help Latinas overcome barriers to initiate MH Tx. Multiple waves of theater-testing and pilot testing showed symptomatic, untreated Latinas reported high identification with the Latina lead-character and high trust of the Latina nurse-therapist character; all was developed using composite data from past qualitative studies with Latinas. Using transmedia (storytelling across multiple digital platforms), the story expands beyond a traditional episode through character-driven, interactive bonus videos and social media posts of the characters that are also psychoeducational plus a character’s blog with links to free MH resources and recommendations of local clinics offering low-cost Tx plus Tx locator. A Latinx cast, director, and script writer enhance socio-cultural acceptability and bring Latinas back to re-watch videos. Access is discreet via smartphone, tablet, or computer. Guided by Ajzen’s Theory of Planned Behavior and supported in feasibility and efficacy testing, we identified targets that mediate help-seeking behavior change including attitudes, propensity to seek help, and stigma. For this 2-arm RCT, Aim 1 is to determine the effectiveness of the Catalina experimental transmedia web-based app compared to a control condition (informational MH videos but no story, links to publicly available MH websites, and Tx locator for low-cost options) across 9 months (baseline, 1, 5, and 9 months) to catalyze therapy or pharmacotherapy initiation. Both groups will receive monthly reminder messages via text or email. Consent procedures allow for verification from medical records if MH Tx appointments were made, kept, or missed and if prescriptions were filled. We hypothesize that, compared to controls, a significantly greater proportion of the Catalina group will initiate MH Tx during the 9-month study. Aim 2 is to test the mediation role of the targets (help-seeking attitudes, propensity to seek help, and stigma) across 9 months to initiate MH Tx in Catalina vs. control groups. The sample will include 876 English and Spanish speaking Latinas, age 18 and older, who are untreated but symptomatic for depression and/or anxiety (>10 on PHQ8 or GAD7) and who have internet access. All aspects of the study are done online. If hypothesized relationships are supported this study has potential for high impact on English and Spanish spe...