This paper derives a new powerful test for mediation that is easy to use. Testing for mediation is empirically very important in psychology, sociology, medicine, economics and business, generating over 100,000 citations to a single key paper. The no-mediation hypothesis $H_{0}:\theta_{1}\theta _{2}=0$ also poses a theoretically interesting statistical problem since it defines a manifold that is non-regular in the origin where rejection probabilities of standard tests are extremely low. We prove that a similar test for mediation only exists if the size is the reciprocal of an integer. It is unique, but has objectionable properties. We propose a new test that is nearly similar with power close to the envelope without these abject properties and is easy to use in practice. Construction uses the general varying $g$-method that we propose. We illustrate the results in an educational setting with gender role beliefs and in a trade union sentiment application.