The following inclusion criteria will apply:  Right-handed participants between the ages of 18 and 50 years inclusive. Able to provide informed written consent Fulfil Diagnostic and Statistical Manual of Mental Disorders (5th Edition) (DSM-5) criteria for a primary diagnosis of current single or recurrent episodes of MDD of at least moderate severity but without psychotic features as defined on the MINI 7.0. Positive and primary diagnoses on the MINI 7.0 will be subject to confirmation at clinical interview by a psychiatrist. 17-item HAM-D score ≥ 18. Have failed to respond to 2 or more antidepressants prescribed at minimum effective dose for at least 6 weeks OR at least 1 antidepressant prescribed at the minimum effective dose for at least 6 weeks AND a course of evidence-based psychotherapy given for at least 6 sessions. The subject is off all drugs likely to interact with glutamate or opioid system at least 14 days before starting the study (antipsychotics, anticonvulsants, mood stabilisers, gabapentinoids, opiates, opioid agonists/antagonists/combinations and stimulants). There are two exceptions. The first is any regular antidepressant(s) the subject may be taking (apart from MAOIs that are not permitted). Second, is short acting benzodiazepines or hypnotics for distressing anxiety or insomnia (up to 72 hours prior to each MRI scan). The subject has a resting pulse ≥51 bpm and ≤100 bpm at the screening visit. For subjects in good physical condition, the lower limit is ≥45 bpm. The subject has a resting systolic blood pressure ≥91 mmHg and ≤140 mmHg and a resting diastolic blood pressure ≥51 mmHg and ≤90 mmHg at the screening visit. An out-of-range resting systolic blood pressure may be repeated once if a medically valid reason is present, for example, the subject suffers from white-coat hypertension or has just come from low outdoor temperatures or experienced stress (e.g. late arrival). The medically valid reason must be documented and signed by the investigator. Body Mass Index (BMI) of 18 to 30 kg/m2 inclusive and a total body weight 50-100 kg at the screening visit. Able to attend interviews and tolerate MRI scanning procedures.  The following exclusion criteria will apply:  Diagnosis of bipolar disorder or psychotic disorder on the MINI 7.0. Positive diagnoses on the MINI will be subject to confirmation at clinical interview by a psychiatrist. A first-degree relative with bipolar disorder, schizoaffective disorder, or schizophrenia, with the potential participant younger than 33 years (ie, still at age of risk for a psychotic disorder). Personal history of a ≥ 1 suicide attempt in the past year, or active ideation with plan and intent defined using the C- SSRS ("Suicidal Behaviour" section and question 5) alongside confirmation at clinical interview with a psychiatrist. Diagnosis of drug or alcohol dependence syndrome (defined as meeting DSM-5 criteria for any dependence syndrome) on the MINI 7.0. Positive diagnoses on the MINI will be subject to confirmation at clinical interview by a psychiatrist. History of IV drug use. Current recreational use of ketamine. A positive urine drug screen on or after the screening visit during their active involvement in the study for ketamine, opiates, methadone, cocaine, amphetamines, benzodiazepines or cannabinoids. History of nonresponse or intolerance to ketamine. Significant uncontrolled physical illness particularly if it may affect the brain or glutamatergic system (blood dyscrasias, lymphomas, hypersplenism, endocrinopathies, renal failure or severe chronic obstructive lung disease, autonomic neuropathies and active malignancy). Significant history of cardiovascular or cerebrovascular illness which may compromise the safety of ketamine use (myocardial infarction, heart failure, valvulopathy, stroke or transient ischaemic attack). Subjects with allergies and sensitivities to ketamine or similar compounds will also be excluded. Inability to provide a screening blood sample, urine sample or electrocardiogram. Biochemical abnormalities (defined as falling outside the normal reference range) as evaluated by a full blood count, and full biochemistry profile. Biochemical abnormalities must also be determined as clinically significant by a medical doctor to fulfil the criterion for exclusion. Electrocardiographic abnormalities, defined as any abnormality that is not normal sinus rhythm and determined as clinically significant by a medical doctor. Women of childbearing potential not using adequate contraception. Pregnant or breast-feeding women. Any previous neurosurgery or neurological disorder, including epilepsy History of head injury resulting in unconsciousness lasting at least 1 hour Any contraindications for MRI Use of compounds that may be affected by the use of ketamine (diazepam, warfarin, carbamazepine, phenytoin, theophylline and levothyroxine). Unwilling or unable to comply with the Lifestyle guidelines.  The following will be reasons for exclusion from analysis:  Excessive head motion on resting state scan (Framewise displacement of 0.5mm on >20% of scans) Images not passing quality control as set out in the analysis plan Any side effects during the scan which could potentially impact interpretability of findings (e.g. nausea)