Watch Paul Shaffer's Uproarious Return to 'Late Night'

David Letterman's former bandleader partners with Jon Batiste and Stay Human, Valerie Simpson for "Ain't No Mountain High Enough"

David Letterman's former bandleader Paul Shaffer returned to 'The Late Show' for a performance of "Ain't No Mountain High Enough."

Paul Shaffer and the World's Most Dangerous Band returned to the Ed Sullivan Theater Monday for a performance of Marvin Gaye and Tammi Terrell's "Ain't No Mountain High Enough" on The Late Show With Stephen Colbert. David Letterman's former bandleader partnered with Colbert's in-house outfit, Jon Batiste and Stay Human, and split vocal duties with R&B star Valerie Simpson.

The two Late Show bands combined to create a massive orchestral pop sound filled with sweeping strings, disco guitars and jazzy horns. Meanwhile, Shaffer's growl served as a perfect foil to Simpson's sultry diva vocals, and the pair deftly traded lyrics, united on harmonies and even took turns playing the grand piano.

Shaffer sat down for an interview with Colbert – and promptly chastised the host for picking a new Late Show theme song. "I wrote the theme of the David Letterman show and it was played every night for all of those 33 years, and you have taken that income out of the mouths of my children!"

The former bandleader reflected on his lengthy Late Show tenure and praised the new bandleader, Batiste, specifically the musically complex noodling the jazz pianist plays to accentuate a Colbert joke. Shaffer also spoke about reuniting the World's Most Dangerous Band and cutting their new self-titled album, which arrived in March.