Like living organisms, cancer cells require energy to survive and interact with their environment. Recently, investigators demonstrated that cancer cells can hijack mitochondria from immune cells. This behavior sheds light on a pivotal piece in the puzzle of cancer, the ‘dependence’. This article illustrates how new, functional mitochondria help cancer cells’ survival in the harsh tumor microenvironment, immune evasion, and progression. In addition, we postulate how blocking the routes cancer cell applies to boost its mitochondria content and function can improve the treatment outcomes of radiotherapy, chemotherapy, and immunotherapy. Besides, this article provides a new theory in oncology, the ‘energy battle’ between cancer and immune cells. It alludes that each party with a higher energy level can win the battle. This theory explains cancer biogenesis and provides novel insights to improve treatment outcomes.