Main inclusion criteria:

          -  Patient is ≥18 years of age

          -  Patient has pathologically confirmed, definitively diagnosed, advanced (not able to be
             treated with surgery or radiotherapy) or metastatic NSCLC and has not been treated
             with systemic anticancer therapy for metastatic disease.

          -  Patient must have a documented RET-fusion

          -  Patient has measurable disease based on RECIST 1.1 as determined by the local site
             Investigator/radiology assessment.

          -  Patient has an ECOG PS of 0-1.

          -  Patient should not have received any prior anticancer therapy for metastatic disease.

               -  Patients can have received previous anticancer therapy (except a selective RET
                  inhibitor) in the neoadjuvant or adjuvant setting but must have experienced an
                  interval of at least ≥ 6 months from completion of therapy to recurrence.

               -  Patients that received previous immune checkpoint inhibitors in the adjuvant or
                  consolidation following chemoradiation are not allowed to receive pembrolizumab
                  if randomized in Arm B

          -  Patient is an appropriate candidate for and agrees to receive 1 of the Investigator
             choice platinum-based chemotherapy regimens if randomized to Arm B.

          -  Patient provides signed informed consent to participate in the study.

        Main exclusion criteria:

          -  Patient's tumor has any additional known primary driver alterations other than RET,
             such as targetable mutations of EGFR, ALK, ROS1, MET, and BRAF. Investigators should
             discuss enrollment with Sponsor designee regarding co-mutations.

          -  Patient previously received treatment with a selective RET inhibitor.

          -  Patient received radiotherapy or radiosurgery to any site within 14 days before
             randomization or more than 30 Gy of radiotherapy to the lung in the 6 months before
             randomization.

          -  Patient has a presence of Grade 2 or worse interstitial lung disease or interstitial
             pneumonitis, including radiation pneumonitis within 28 days before randomization.

          -  Patient has CNS metastases or a primary CNS tumor that is associated with progressive
             neurological symptoms or requires increasing doses of corticosteroids to control the
             CNS disease. If a patient requires corticosteroids for management of CNS disease, the
             dose must have been stable for the 2 weeks before Cycle 1 Day 1.