Hear Neil Patrick Harris Sing in a Sex-Swapping Episode of Adventure Time (VIDEO)

Some of the best shows on television right now aren't great because they're breaking the rules, they're great because they have no rules. I'd put FX's Louie and NBC's Community on that list, but coming in like a bullet is Cartoon Network's Adventure Time, the brilliant 'toon that brings smiles to toddlers, chic geeks, and giggly stoners everywhere.

The next new episode airs Monday, September 5, and it's quite the special treat. Near as I can tell, it's set in a parallel universe where boys are girls and girls are boys and dogs are cats ZOMG! Finn the Human is now Fionna the Human, Princess Bubblegum is Prince Gumball, and Jake the Dog is Cake the Cat. Even Lady Rainicorn looks more masculine.

If that's not enough to convince your stubborn self to tune in to the episode, which is called "Adventure Time with Fionna and Cake," Neil Patrick Harris guest-stars as Prince Bubblegum and does one of the 1,384 things he does best: siiiiiiiiiiiing! Here's an advanced look at NPH serenading Fionna:

And in case you missed this Adventure Time teaser from Comic-Con this year, here's the kick-ass revamped opening sequence plus the opening minute or so of the episode:

Totally algebraic, dude.

... Cake's voice: Like or dislike?
... Does this mean no Tom Kenny or John DiMaggio?
... What's cooler, a dog who plays the viola or a cat who plays the xylophone?
... How awesome is it when Fionna slices up those flying eyeballs?
... Is it just me, or does it look like Candy Corn Man is still a man?

Follow TV.com writer Tim Surette on Twitter: @TimAtTVDotCom

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