For those who don't follow Homestuck:

The kids seen are playing a game that affects their entire universe. The boy with glasses (John) is a "server" player for the "client" player, the girl with glasses (Jade). Dave can alter Jade's real-life surroundings through the game. Jade has just entered the game, and her first task is composed of three things: 1) "prototype" an object or creature, 2) break an object provided by the game that is related to her surroundings (in this case a piñata), and 3) avoid a meteor created by the game that will destroy her in 6 minutes if she fails to do 1 and 2 by then. "Prototyping", mentioned earlier, turns the object or creature into a sprite which will aid the client player throughout the game. It also causes certain enemies (including the final boss) to have some of that object's attributes.

A troll (Vriska) watching these events from a different universe causes the boy with glasses to fall asleep using her mind powers. The boy was going to prototype a doll to prevent the enemies from gaining any power, but is prevented by the Vriska's powers. Enter "Bec", the weird super-powerful dog-like thing that is Jade's guardian. Bec, being unimaginably intelligent, knows that nothing being prototyped is the worst possible situation, so he prototypes himself by entering the floating flashing orb. He then dramatically turns into Becsprite (a moment during which every MSPA reader in the world simultaneously screams OH SHIT because this means that the enemies will have Bec's abilities to warp spacetime). Bec then destroys the impending meteor (creating some sort of explosion on Earth in the process).

In the meantime an entirely different battle is happening between Bro (tall guy with shades and a sword) and Davesprite (floating orange guy with shades and sword) both versus Jack Noir (dark dude with a jester's hat and scarred eye). Bec helps Jade destroy the piñata by redirecting her bullet. The piñata's destruction initiates two things: 1) Jade's island is removed from its place and put into the game universe, and 2) the rings of the Black and White Kings (chess analogies of the feuding light and dark kingdoms mentioned earlier) now give its wearer the powers of Bec. Jack Noir possesses one of the rings and is at this point given Bec's abilities and appearance. He creates a 'rift' in spacetime to enter the trolls' universe, where they are about to claim their prize for winning their own (separate) game session, ultimately preventing then from entering the orb. The robot troll (Aradia) uses her powers to send the group into an abandoned facility on a nearby meteor to save them from the human session's Jack Noir (which is entirely different from her own game's Jack Noir, who never became nearly as powerful).

One of the trolls (Karkat) wakes up in yellow garments; he is asleep in the aforementioned facility, but his dream-self is now awake on what was once the light kingdom's planet's moon. He sees the human session's Jack Noir destroy the moon and planet (both yellow and filled with buildings everywhere), which also destroys every troll's dream-self (as those selves were all on that moon). It ends with the girl with glasses falling in some sort of void; the implications of this are still unknown.

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TL;DR: when the dog thing enters the orb, that's REALLY REALLY BAD and is probably one of the most important events in Homestuck.

Violet said:
I have no idea what's going on here... but for most of the video, I couldn't shake the single thought: "Was that a Bill Cosby laptop?"

Creation of the Cosbytop: http://mspaintadventures.com/?s=6&p=002965

I should clarify part of my summary above: Jack Noir goes BACK IN TIME when he creates the rift. Vriska does her thing AFTER Jack Noir has entered her universe and BEFORE he has left the humans' universe. It's a self-fulfilling paradox.