Adventure Time Theme Song

The "Adventure Time Theme Song" and opening title sequence features all of the main characters and also some of the minor characters. The theme is sung and played by series creator Pendleton Ward. It has the same lyrics as the animated short but there are some slight variations.

Summary
The opening title sequence starts with a view of a post-apocalyptic wasteland that features a rat inside of a broken television and several undetonated nuclear bombs. Next, in a snow-covered biome, are shown penguins, including Gunter and two other penguins hugging (with a tiny red heart floating above their heads) that make a few quick quacking sounds, and a Snow Golem sitting on a grassy hill nearby. Then the rest of the Ice Kingdom appears including Ice King flying next to his lair, spinning around to face the screen while activating his Ice Powers with his hands. It shows the Candy Kingdom and a large crowd of Candy People laughing and dancing in the middle of the road. After that, the camera quickly scales Princess Bubblegum's Castle, eventually passing the clouds to where we can see Princess Bubblegum as she smiles and waves while riding on the back of Lady Rainicorn. The camera then pans to the right, and zooms into a mountain range showing Marceline playing her Fender Precision Bass while sitting outside of her cave. She turns and hisses at the audience while showing her pointy teeth. Then outside of Finn and Jake's treehouse is the two-headed duck. The ending shows Finn and Jake's Tree Fort and then the two of them inside. BMO is seen sitting nearby. Finally, they pound their fists together and a loud smashing sound is heard as the theme song begins. This entire ordeal occurs approximately within the span of 12 seconds. When the show's title is seen, Finn (without his green backpack) and Jake float up using feathered wings sticking out from their backs while holding a white piece of cloth that says with Finn and Jake. Then, they are seen going somewhere as the lyrics go "C'mon grab your friends," and then Jake winks when the lyrics "Jake the Dog" come up, and Finn seems to bark and stick his tongue out when the lyrics "Finn the Human" come up. The sequence ends when Finn and Jake reach the top of a rocky mountain that they were climbing. Finn gets to the top and strikes a victorious pose while Jake lies down near Finn as the show begins.

Lyrics
The lyrics with ukulele chords to the theme song are as follows:

Characters
The following characters can be seen in the opening sequence:
 * Rat
 * Snow Golem
 * Penguins
 * Ice King
 * Candy People
 * Princess Bubblegum
 * Grape Popsicle Guy
 * Starchy
 * Mr. Cupcake
 * Gingerbread Muto
 * Sugar Cube
 * Lollipop Girl
 * Gumdrop Lass 1
 * Gumdrop Lass 2
 * Chocoberry
 * Mr. Candy Cane
 * Gumball Guardians
 * Raisin
 * Candy Corn Guy
 * Root Beer Float
 * Lady Rainicorn
 * Marceline
 * Two-Headed Duck
 * Finn
 * Jake
 * BMO
 * Tree Trunks
 * Bats

Early versions
There are many early versions of the title sequence that didn't get used. Many recognizable faces appear, including a worm, Ice King, Iceclops, Marceline, Princess Bubblegum, Lady Rainicorn, Phil, Tree Trunks, Gork, Wendy, and even the Snail.

Alternate versions
"Fionna and Cake" is the first episode (the second is "Bad Little Boy") to feature an alternate version of the opening theme, performed by Natasha Allegri instead of Pen. It also featured many of the show's characters, re-drawn as the opposite gender. The only lyric change is "Cake the Cat and Fionna the Human" for "Jake the Dog and Finn the Human."

Trivia

 * During the post-apocalyptic wasteland scene, you can see a gray arm with purple spots and a bite mark coming out of the tree that has axe and arrows in it. It has been confirmed to be a zombie arm.Screen Shot 2013-09-10 at 9.35.44 PM.png
 * In Issue 1 of the Adventure Time comic, it can be seen that Jake was actually filming the opening sequence.
 * Before Marceline opens her mouth, her fangs are blunt. This is also seen in the episode "Daddy's Little Monster."
 * In both versions, (Fionna and Cake and Finn and Jake) a two headed duck can be seen right in front of the tree fort.
 * In the Spanish (Latino) version, when the singer says Jake's name it sounds as if she is rushing. This might be because the song and pictures were going in a fast pace.
 * Only two different versions of the intro are seen so far, the first is in "Fionna and Cake" (and "Bad Little Boy") and the second is in "A Glitch is a Glitch."
 * The Arabic, Bulgarian, Danish, Hindi, Japanese, Polish, Russian, Spanish (Latino) and Swedish versions of the intro use more than one voice when singing the theme song.

Production notes

 * A character peeping over the balcony of the Candy Castle can be seen briefly. According to Pendleton Ward, this is a cameo by Gingerbread Muto, the Candy Person version of storyboard artist Adam Muto. Pen says he puts Adam in most of his films because it "riles him up."
 * The final shot with Finn and Jake posed on a mountain was inspired by the fantasy art of Frank Frazetta.
 * The beginning of the sequence, with the rising musical tone and flying over the landscape was inspired by the opening sequences of The Simpsons and Pee-wee's Playhouse.
 * Pen states: "If you listen carefully to the recording, you can actually hear Derek Drymon typing on a keyboard in the bit when Jake is walking... a little clickity clack in that wide shot when his legs are stretched out. I recorded the lyrics for the opening title in the animatics room where we have this little crummy microphone JUST so that we could add it to the titles and submit it to the network. Later, we tried re-recording it and I didn't like it... I only liked the temp one."

Differences between the two intros (Original and sex-swapped)

 * When Fionna is holding a leaf on a stick, Cake jumped in Fionna's face, while Jake just jumped in Finn's face without Finn holding anything.
 * At the end of the "Fionna and Cake" version there are no skulls on the ground.
 * Cake curls up and falls asleep instead of grabbing Fionna's leg and smiling, as Jake did to Finn.
 * The sky doesn't change color as it does in the original; it is instead a bright shade of bluish-green (a color not used for the sky in the original).
 * The sky is less cloudy in the "Fionna and Cake" version.
 * The sword sticking out of the mountain to the right of the characters is different.
 * The mountains in the background are different.
 * When Fionna is riding Cake in the Ice Kingdom, a xylophone can be heard instead of a keyboard.
 * Marceline's eyes are outlined, while Marshall Lee's aren't.
 * When Fionna opens her mouth near the end, no teeth are shown. Finn's are in the normal version.
 * Finn's mouth is open while riding on Jake in the Ice Kingdom, while Fionna's isn't. Fionna is also looking downward while Finn is looking forward.
 * When Finn and Jake fist bump, Jake's eyes are focused on the fists, but Cake's eyes are focused on Fionna.
 * Marceline's tongue is shown shaped as a snake's tongue but Marshall Lee's tongue is more of a human tongue.