Research has demonstrated the ability to identify and treat individuals at high risk of developing psychosis. It is possible to use a similar strategy to identify people who have an emergent risk of bipolar disorder (BD). Interventions during the early phase may improve outcomes and reduce risk of transition. Criteria have been established to identify individuals considered to be at high risk for developing BD, also known as Bipolar At Risk (BAR). Offering a psychological intervention may provide the possibility of prevention. We are conducting a large multicentre, rater-masked randomised controlled trial ISRCTN (ISRCTN13363197, registered on 25th January 2023) with two parallel arms comparing cognitive behaviour therapy (CBT) for young people meeting BAR criteria (CBTBAR) + Treatment as Usual (TAU) vs. TAU alone. Recruitment of 338 participants will be required to test our efficacy hypotheses: CBTBAR + TAU (compared to TAU alone) will lead to improvement in mood swings, a reduction in the likelihood of transition to BD, and improvements to functioning and quality of life. Our mechanistic hypothesis is CBTBAR + TAU causes improvement in mood swings due to the reduction of extreme positive and negative appraisals of internal states which in turn improves subsequent behaviours used to control mood and then internal states. Our trial will explore the perceived mechanism of change via this novel intervention (CBTBAR) and if the approach can be implemented within current services in the UK.