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A man in his mid-70s presented with haematuria and haematospermia and experienced a multiyear diagnostic delay due to initially benign impressions. A diagnosis of Gleason 8 prostate adenocarcinoma was eventually rendered, despite consistently normal prostate-specific antigen levels. A prostate-specific membrane antigen-positron emission tomography scan, negative for prostatic uptake, unexpectedly revealed a lung lesion, subsequently confirmed as metastatic adenocarcinoma. Expert pathology review at a tertiary cancer centre, using comprehensive immunohistochemistry—including paired box gene 8 (PAX8), cancer antigen 125 (CA-125) and hepatocyte nuclear factor 1 beta (HNF1Beta) positivity alongside negative prostatic markers—established a final diagnosis of metastatic clear cell seminal vesicle adenocarcinoma invading the prostate. Despite therapies targeting a checkpoint kinase 2 (CHEK2) mutation (a poly(ADP-ribose) polymerase (PARP inhibitor)), followed by taxanes, platinum-based chemotherapy and radiosurgery for brain metastases, he ultimately succumbed to a stroke linked to his malignancy.