Breast cancer is the most prevalent type of cancer and second leading cause of death in women. Among others, the triple negative breast cancer (TNBC) is the most invasive as it has the highest recurrence and death rates with no targeted therapeutic available thus far. Epidermal Growth Factor Receptor (EGFR) is one of the important targets as more than fifty percent of the TNBC overexpress it but all the therapies designed against it have failed to show significant results. The juxtamembrane domain of EGFR has been explored comparatively recently and has been used to design a decoy peptide with the anticipation to affect the EGFR downstream functions. Previous research has shown it to cause cell death in cancer cells. This study is aimed towards deciphering the mechanism of action of the stapled form of this decoy peptide-SAH5. It presents evidence that the peptide leads to an immediate intracellular calcium release from the Inositol 1,4,5 triphosphate on the endoplasmic reticulum, an inhibition of which can rescue SAH5 induced cell death. The study also demonstrate that the peptide is able to increase the production of Reactive Oxygen Species (ROS) in mitochondria, part of which is triggered by the peptide-induced calcium release.