A flexible plunger boot used with a plunger to unclog a drain includes a coupling section, bulbous seal, and periphery seal. The coupling section couples to the bottom end of the plunger. The bulbous seal has a hollow spherical shape with a maximum outside diameter that is larger than a diameter of a drain that the bulbous seal is inserted into and deforms inwardly to create an interference fit with the walls of the drain. The periphery seal has an annular ring shape that is smaller in diameter than the maximum outside diameter of the bulbous seal but larger in diameter than a drain opening whose diameter is too small for the bulbous seal to fit into. Whenever the periphery seal is pressed down against the surface surrounding the drain opening it flattens against the surface to create a mechanical seal with the surface.