This episode of Supergirl imports one of the most popular aliens in the DC Comics canon: the magical imp Mr. Mxyzptlk, a childlike, super-powerful being from another dimension, with the power to pretty much do anything he wants. (Think of him as a precursor of Q from Star Trek: The Next Generation.) Tonight he shows up on Supergirl, not to crush her – but with a crush on her. Which explains why Supergirl finds herself, suddenly, not in her red and blue uniform, but in white – as in, a wedding dress. The episode’s title: “Mr. & Mrs. Mxyzptlk.”