Every heartbeat leaves a trace on an ECG, but can we use that trace to reconstruct what is happening inside the heart? In my research, I combine mathematical modelling, computer simulations, and ECG measurements to build digital twins of the heart: personalised virtual models that capture the electrical activity of an individual heart. In this talk, I will take you on a journey from a simple ECG signal to the mathematics behind a virtual heart, introducing some of the ideas of cardiac electrophysiology along the way. I will also share how I ended up working at the intersection of mathematics, computing, and medicine, and what my journey through academia has looked like.