Who's been messing up everything? It's been Agatha all along!

If this little jingle has been stuck in your head for a week, chances are you're not alone.

The song from "WandaVision", the hit Marvel Studios show currently streaming on Disney Plus, has topped the iTunes soundtrack chart and racked up over 1.7 million streams on Spotify.

Maybe it's the effervescent Kathryn Hahn, or the equally brilliant performances of Elizabeth Olsen and Paul Bettany as Wanda Maximoff and the Vision, or simply because Marvel fans have been starved for content. "WandaVision" is a bona fide hit.

But if you're still confused about everything that went down, read on.

Please stand by

The events of "WandaVision" takes place shortly after "Avengers: Endgame".

There were more than a few fans who laughed when Wanda snarled at Thanos that he took everything from her, and his honest reply.

Even though this was a different version of Thanos, she was understandably furious. Wanda had lost her parents, and then her twin brother Pietro during "Age of Ultron".

She fell in love with Vision, but was forced to destroy him during the events of "Infinity War". Then she watched Thanos reverse time itself and kill Vision to get his Mind Stone.

That's a LOT of emotional weight for one woman to bear. Grief -- and processing it properly -- is the heart of "WandaVision".

Spoilers ahead

After being told she couldn't bury Vision properly, Wanda visits the town of Westview, New Jersey where Vision had bought a plot of land, intending to build a house where they could grow old together.

Overwhelmed by emotion, Wanda used her powers to create a facsimile of Vision, and a neighbourhood where everything was "perfect" and under her control.

Inspired by the old American tv shows she used to watch as a child, the neighbourhood was modelled after the "Dick Van Dyke" show of the 1950s, complete with a monochrome set up and a live studio audience.

As things progress, the setting changes, inspired by "Bewitched" in the 1960s, "The Brady Bunch" in the 1970s, "Family Ties" of the 1980s, "Malcolm in the Middle" of the 2000s and "Modern Family" of the modern era.

More people and new locations show up, as Wanda expends her power. She and Vision have twin boys, Tommy and Billy, who have powers of their own and age up from babies to 10-year-olds within hours.

It is eventually revealed that the couple's "nosy neighbour" Agnes is actually Agatha Harkness, a centuries-old witch who wants to learn the secret of Wanda's powers. A government-type organisation called SWORD is also observing the events, but is led by a director with sinister intentions.

Wanda defeats Agatha and sacrifices her "perfect life" with Vision, Tommy and Billy to free the people who were under her mind control.

How did Wanda get so powerful?

Wanda gained her telekinetic and telepathic powers after undergoing experiments with the Mind Stone by Hydra, a villainous organisation.

But Agatha tells Wanda that she is naturally gifted with magic, even before the experiments. She is the Scarlet Witch, an immensely powerful sorceress who can use Chaos Magic.

This is seen in her flashback to her childhood, where she instinctively uses magic to defuse a bomb.

But the Mind Stone did help -- Wanda explained that she could create a version of Vision because they are both linked to the Stone.

What is SWORD?

In the comics, SWORD stands for Sentient World Observation and Response Department, a counterpart to SHIELD which monitors threats from outer space. Here, the "W" stands for "Weapon", the "D" for "Division", and it appears to be tracking anomalies.

SWORD enlists the help of some familiar faces like Darcy Lewis from the Thor movies and FBI Agent Jimmy Woo from "Ant-Man and the Wasp".

Jimmy Woo and Darcy Lewis #CharactersThatNeedASpinoff pic.twitter.com/U9LfyZHitf

— Dan Levey (@iamdanlevey) February 21, 2021

The series also brings back Monica Rambeau, who was a young girl in the "Captain Marvel" movie. Here she's older and is working for SWORD, which was set up by her mother Maria.

The final episode of "WandaVision" appears to set up the "Captain Marvel" sequel, with Monica about to go to space.

Who are Tommy and Billy, and what does it mean for the Doctor Strange sequel?

Wanda's twins appear in the comics too, later as members of the Young Avengers team as Speed (Tommy, super-speed) and Wiccan (Billy, magic).

During the final post-credits scene, Wanda hears the cries of her twins, who may not be permanently gone.

The scene also depicts Wanda studying magic through astral-projection, something we've seen before in "Doctor Strange".

#WandaVision#WandaVisionFinale
just Wanda using the Darkhold to search for alternate realities // astral projection easter for Doctor Strange. pic.twitter.com/CZaSxuIDgY

— maximo (@jmbvmts) March 5, 2021

Agatha also mentions Strange, stating that the Scarlet Witch is more powerful than the Sorcerer Supreme, one of Strange's titles.

Wanda is reading the Darkhold, which Agatha calls "the book of the damned". The Darkhold has shown up before, in the Agents of Shield tv show.

In the comics, it is a collection of evil spells written by the Elder God Chthon -- also known as the God of Chaos.

Remember that before the pandemic hit, the "Doctor Strange" sequel, which features Wanda, would have been released in May 2021. That would have seen "WandaVision" setting up the movie.

This means it's likely that the Scarlet Witch, the Darkhold, and the twins could all play a part in the "Doctor Strange" movie, now slated for a March 2022 release.

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Top image from FilmsActu Facebook page and Jobarfcreepy Twitter.