This is a case of a patient who presented to the emergency department with acute abdominal pain due to bowel obstruction. An extended right hemicolectomy with ileosigmoid anastomosis due to an obstructing mass on the splenic flexure was urgently performed. During operation, liver and peritoneal lesions were detected and samples were also sent for histological analysis. Pathology report was consistent with poorly differentiated mucinous adenocarcinoma with signet ring cells; peritoneal lesions were confirmed histologically as metastatic. Genetic testing revealed the BRAFV600E mutation and mismatch repair deficiency (dMMR). After progressing on 1st line chemotherapy, the patient has a continuing and long-lasting partial response to 2nd line treatment with pembrolizumab.