How I Met Your Mother star Neil Patrick Harris lends his voice to a special episode of Cartoon Network's hit animated series Adventure Time.

Neil Patrick Harris stars as Prince Gumball in Adventure Time with Fionna and Cake, a gender-swapping, parallel universe episode featuring the female "Fionna" and her cat "Cake."

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The episode premieres Monday, Sept. 5 at 8 p.m. (ET/PT). No appearance by Neil Patrick Harris would be complete with out a song.

Adventure Time introduces viewers to unlikely heroes Finn and Jake, buddies who traverse the mystical Land of Ooo and encounter its colorful inhabitants. The best of friends, our heroes always find themselves in the middle of heart-pounding escapades. Finn, a silly boy with an awesome hat and Jake, a brassy dog with a big kind heart, depend on each other through thick and thin.