Altar of Gravity Falls, Gideon's Room

It was a very dark day for the pines family, but specially for Mabel, who's greatest nightmare was about to come true. In contrast, Gideon was having the time of his life as he ready for the bgi wedding, and what best way to show how happy he is by singing?

This day is absolutely perfect,
The greatest of my dreams has finally come true,

As he continued to sing he started to dance with a mannequin that was wearing an elegant red dress and who's figure and appearance were disturbingly similar to Mabel's.

I have Mabel as my bride,
And no Pines will intervene,

Throwing away the mannequin, which fell through the window and fell to the ground, Gideon walked towards a mirror with an evil grin, with the sudden appearance of a third eye in both his suit and his forehead.

And once it's done,
I'll make sure they're gone!

At the last verse the lights turned off, with the only thing visible being Gideon's eyes and the third one, which was surrounded by blue flames.

Underneath the Gravity Falls Cave, Diamond Tunnel

Meanwhile the rest of the mystery shack team were running around the tunnel, trying to find a way out of there. For the first part of the first verse the shack team were running inside a diamond each, Wendy had a pink one, both Soos and Stan had a gray one and Dipper a blue one.

This day is a complete disaster,

For the second part of the first verse everything returned to normal and the team continued to run until they stopped to catch their breath close to some diamonds, which the reflection showed what happened to the team after their failed attempt to rescue Mabel.

Wendy had a black eye and Soos, Stan and Dipper were badly beaten up.

We can't believe it could get this worse,

The team sat on close to said diamonds as Wendy, Soos and Dipper sung the next verse.

Even though we tried to save the day,
We failed in such an epic way,

When it was Stan's turn, he got up and looked at the ceiling, where he saw Gideon's face in one of the stalagmites.

When we return,
Gideon's gonna pay!

Back at the Altar

Gideon was trashing the possible dresses Mabel could wear while his assistants, Candy and Grenda, watched in horror. After thrashing all of said dresses, he turned to the girls and went walking to them, continuing his singing when he arrived.

I don't care if she's on bliss
Nor if she has a parti pris
My only conditions are this:

Turning around and walking to a painting of an incredibly elegant Gideon, with the words "first class" written underneath in red painting, Gideon told the two girls his conditions.

Since I am someone of first class,
Make sure she's as sweet as the kvass,
Today she will no longer be a lass!

Before the next part he turned around and went walking to Candy and Grenda in a menacing tone, with the girls taking a step backwards.

I love Mabel,
She resides on what's left of my humanity,

As the lunatic boy sung the next part a close up of his three eyes, each surrounded by blue flames followed.

And she'll be my wife for the eternity!

Back at the tunnel

Continuing to run through the tunnel the team spotted an abandoned mine cart with some rocks blocking the way. Without hesitation Wendy, Soos and Stan tried to remove the rocks, but they were too heavy.

We must escape ASAP
And set Mabel free,
Of this horrible destiny!

As the trio continued to try remove the rocks, Dipper briefly looked at the amulet he had on his neck. The amulet was like Gideon's, and remembering what that did, Dipper got an idea and smiled.

Marrying Gideon
She won't last
The experience will break her heart

Dipper put his hand on the amulet and used its mystic power to telekinetically remove the rocks, by which everybody got on the mine cart and Dipper pushed it before going inside.

Oh Mabel,
we're coming to your aid!

As the mine cart continued to move and increase speed , the cart went through a stop signal and the shack team flew of the cart. When they realized they weren't falling to their doom, everybody opened their eyes to see Dipper holding the amulet with his hand and the shack team being surrounded by a blue aura as they flew.

Altar of Gravity Falls

Meanwhile, Gideon was waiting at the altar for the moment Mabel arrived. Almost the entire town of gravity falls was present at the occasion, with Gideon's mother crying out of joy while Bud was sitting at their side.

Hearing the door opening loudly, Candy and Grenda arrived smiling in a scared way throwing flowers as Mabel, who had her hands handcuffed, her mouth covered by a bandanna and an automatic voice over attached to her neck was forced to walk forward as if she didn't, Gideon's bodyguards would do... something to her.

At last, the moment has arrived,

As Gideon sung the next part, we see Mabel closing her eyes as she start to cry.

To marry the perfect, submissive wife!

Having left the cave a while ago the shack team were now running to the altar, with the only one singing being Dipper.

We have to hurry to the wedding,
and stop him from putting the ring,
Or else Mabel will be-

Everything went dark as Gideon appeared with an evil grin.

"Mine, and only mine!" finished Gideon.

He started to laugh like a maniac as the song ended, he vanished and his third eyes glowed a blue color before disappearing as well.

The End