Forget all about Batman vs. Superman immediately, because the only superhero-powered clash of the titans worth thinking about anymore is Poppins vs. Poppins: Supercalifragilisticexpiali-Justice. EW confirmed that Oscar-winner Julie Andrews has an until-now-secret role in Aquaman. Director James Wan's underwater DC superhero epic just happens to be set to go head-to-head at the December box office with Rob Marshall's Mary Poppins Returns, starring Emily Blunt in the magical umbrella-wielding title role made famous by Andrews in her feature film debut.

For Aquaman, Andrews will lend her voice to the character Karathen, an aquatic creature created entirely for the film who is "key" to Arthur Curry's (Jason Momoa) orange-and-green mission to bring peace between the surface world and his native Atlantis.

“We wanted Karathen to have the voice of a classic British actress, albeit somewhat digitally altered,” Aquaman producer Peter Safran said. “And when we found out Julie was interested and available and excited to do it, casting her was a no-brainer...Who knew that Aquaman would have the real Mary Poppins this Christmas?”

Aquaman—which also stars Amber Heard, Nicole Kidman, Patrick Wilson, Yahya Abdul-Mateen II, and Willem Dafoe—surfs into theaters on December 21. Mary Poppins Returns will beat it there by a few days on December 19. May the best Poppins win.

Here is the official synopsis for James Wan's Aquaman:

From Warner Bros. Pictures and director James Wan comes an action-packed adventure that spans the vast, visually breathtaking underwater world of the seven seas, “Aquaman,” starring Jason Momoa in the title role. The film reveals the origin story of half-human, half-Atlantean Arthur Curry and takes him on the journey of his lifetime—one that will not only force him to face who he really is, but to discover if he is worthy of who he was born to be…a king.