Primary cardiac tumours are exceedingly unusual and aggressive; they often develop in younger patients and present with advanced disease. The rarity and heterogeneity of primary cardiac tumours challenge the standardisation of therapeutic guidelines. Undifferentiated primary cardiac spindle cell sarcomas, a distinct subset of primary cardiac sarcomas, are especially unique with <20 cases reported worldwide, the majority of which are of left atrial origin. This article presents a review of the aetiology, pathophysiology, and therapy of undifferentiated primary cardiac spindle cell sarcomas. In conjunction, the authors present a unique case of a woman with hereditary nonpolyposis colorectal cancer (Lynch syndrome) who presented with a primary cardiac spindle cell sarcoma of left ventricular origin; this is the first case of this type and location of cardiac tumour reported in a patient with Lynch syndrome.