Cytotoxic T lymphocytes (CTLs) recognise and kill cognate target cells by releasing the pore-forming protein perforin and pro-apoptotic granzymes, into the synapse formed between the CTL and its target. There, perforin disrupts the plasma membrane of the target cell, allowing diffusion of granzymes into the cytosol, where they initiate apoptotic death pathways. Paradoxically, despite being exposed to the same amount of perforin as the target, the CTL plasma membrane remains intact, and the cell escapes its own toxins unscathed. Using a combination of atomic force and electron microscopy on artificial lipid membranes, we followed the multi-step mechanism of perforin pore formation at the molecular level. Extending this approach to more complex artificial lipid membranes and using immunological assays to study perforin specificity on live cells, we found two protective properties of the CTL plasma membrane: its high lipid order repels most perforin and, in addition, exposed phosphatidylserine sequesters and inactivates any remaining protein. The resulting resistance of CTLs to perforin explains their ability to kill target cells in rapid succession and to survive these encounters. Our findings also imply an unsuspected role of plasma membrane properties in cells that evade immune attack.