The album is the third installment in Fagen's personal trilogy: The Nightfly about youth, Kamakiriad about midlife, and Morph the Cat about endings. As Fagen spoke of the album, “I’m starting to get older and began to think about mortality a little more. My mother died in 2003 and that was a big shock. When your parents start to die off, that’s going to be a revelation. So for me, this album, although it might sound quite cheery, is really talking a lot about death.” PopMatters expanded on this idea, "Lyrically, Morph the Cat is a logical successor to the first two Donald Fagen solo albums, The Nightfly and Kamakiriad. While Nightfly was set nostalgically in the 1950s and 1960s of Mr. Fagen’s youth, Kamakiriad and Morph are present-tense missives from middle age and late middle age, respectively." The Guardian also described the album's theme of mortality as "blackly appropriate that the funky 'Brite Nitegown' lists three encounters with death: a verse each for sickness, murder and drugs. With razor-sharp horns and Prince-like vocals, it's a killer." In 2006, Fagen also commented that his next project (Sunken Condos, released in 2012) will not be related to these three autobiographical releases, and that "it will be something completely new and it won't be part of this trilogy." The album also has distinct political overtones. Fagen said that "Mary Shut the Garden Door", which depicts a frightening alien invasion, was inspired by the 2004 Republican National Convention in Manhattan and contains allusions to those then in power in the United States. “What I Do” has been described as "a dastardly clever conversation between the ghost of Ray Charles and a young Donald Fagen seeking romantic tips".
