The presently disclosed subject matter is directed to a method of treating cancer, such as (but not limited to) metastatic bladder and breast cancer. The disclosed method comprises using two treatment modalities to synergistically treat primary and secondary tumor cells in a subject. The first element of the method comprises administering a therapeutically effective amount of a plasmonics-active metal nanoparticle to a subject comprising a primary cancer and a distant metastatic site, wherein the nanoparticle concentrates at the primary cancer. The method further comprises exposing the subject to photon radiation at the site of the primary cancer. The second element of the disclosed method comprises administering a therapeutically effective amount of an immune checkpoint modulator to the subject. The synergistic combination provides a rapid, safe, and effective treatment of local and distant lesions, better than each modality alone.