A pressure-isolation device has a plug for isolating pressure within a wellbore. The plug can include one or more modular mandrel assemblies partially disposed within each other. Each mandrel assembly can include a sealing element and a shear pin for setting the mandrel section. Each mandrel assembly can also have a collar element, a shear sleeve, a mandrel stop element, and a back-up ring in contact with the sealing element. The plug can further include a bottom receiving element including a knockout plug. The plug assembly can include a top catch element for setting and pulling the plug.