Behavioural Activation Therapy and Esketamine for Resistant Depression

Inclusion Criteria: * English speaking * Ages 18-65 * Participants meeting criteria for major depressive disorder (MDD) or bipolar disorder, depressive episode without psychotic symptoms according to the Diagnostic and Statistical Manual for Mental Disorders (DSM-5). * Participants who have not responded adequately to at least two separate courses of treatment with different antidepressants, each of adequate dose and duration, in the current moderate to severe depressive episode. Exclusion Criteria: * Depression secondary to a stroke, cancer, or other severe medical illnesses. * Pregnant, lactating or of childbearing potential and unwilling to use an approved method of contraception during the study. * A history of intracerebral hemorrhage, vascular disease. * Active psychotic symptoms. * Current and/or recent history (\<12 months) of substance use/dependence (except for caffeine or nicotine) or problematic current alcohol use or dependence as defined by DSM-5 criteria. * A diagnosis of major neurocognitive disorder or a Montreal Cognitive Assessment (MOCA) score \<24. * Active suicidal intent with the absence of psychotic symptoms is not an exclusion criterion, as this is not atypical in individuals with treatment-resistant, and/or severe depression (safety monitoring will be carried out by research personnel/study psychiatrists). * Known history of intolerance or hypersensitivity to ketamine. * Any other condition that, in the opinion of the PI/study investigator(s), would adversely affect the participant's ability to complete the study or its measures. * The participant must not be receiving psychotherapy treatment outside the clinical trial for the duration of the study.