Patients will undergo blinded serial ctDNA screening every 3 months from the point of registration and completion of primary treatment for their triple negative breast cancer. If a ctDNA positive result occurs on or before their 12 month ctDNA screening assessment they will be randomised by the Institute of Cancer Research Clinical Trials and Statistics Unit (ICR-CTSU) in a 2:1 ratio to the pembrolizumab treatment arm or observation arm. The patient and their treating team will only be informed of the randomisation if they are allocated treatment.