Increases in cytosolic Ca2+ levels control many cellular processes, including the one that starts all life: egg activation at fertilisation. The entry of the sperm into the egg triggers a series of Ca2+ oscillations that last for several hours until pronuclear formation at the first interphase. The oscillations restart when the zygote undergoes its first mitosis, but what regulates these cell-cycle-dependent oscillations is unclear. On p. 2513, Karl Swann and colleagues propose that this pattern of Ca2+ oscillations is achieved in early mouse embryos through the nuclear targeting of a sperm-specific phospholipase C (PLC): PLCζ. They show that PLCζ-induced Ca2+ oscillations occur only during M phase, that PLCζ localises to the pronuclei when they form and that, when this localisation is prevented by the removal of the PLCζ nuclear localisation signal or by inhibition of pronucleus formation, the Ca2+ oscillations are prolonged. The authors propose that, at fertilisation, sperm-derived PLCζ enters the egg cytosol, where it initiates and maintains Ca2+ oscillations until pronucleus formation sequesters it away from its substrate.