"Peter Quill is still feeling down after the loss of Gamora, so Mantis and Drax fly to Hollywood, California on the planet Earth, looking for the perfect Christmas present for him: his childhood's hero Kevin Bacon."

Trailer

• Marvel Studios’ Special Presentation: The Guardians of the Galaxy Holiday Special | Official Trailer

** SPOILERS **

... Excerpts from Guardians of the Galaxy Holiday Special: The 5 Biggest MCU Reveals:

1. Star-Lord Has a Sister

Chris Pratt’s Star-Lord is a pretty tragic guy beneath that goofy exterior. He lost his mother at an early age and was raised by space pirates. No sooner did he meet his long-lost father than he discovered Ego was a Celestial hellbent on remaking the universe in his own image. The only family Star-Lord has left is the one he’s built for himself. So it seems only fitting that we learn Peter, like Luke Skywalker before him, had a sister who’s been hiding in plain sight all along.

The reveal that Mantis is also Ego’s child may be a major departure from the comics, but it helps fill in some of the major gaps in her MCU back-story. It explains how she ended up as Ego’s servant, and why he felt compelled to keep her around in the first place. We can infer that, while Mantis wasn’t quite the perfect child Ego had been seeking for all those years, she was powerful enough that Ego didn’t simply want to kill her like all his other abandoned children.

2. Groot Is Almost Grown Up (Again)

3. The Guardians Have Taken Over Knowhere

The Holiday Special reveals two important details about Knowhere in this post-Blip landscape. One, we now have confirmation that The Collector is still alive and wasn’t killed by Thanos (something actor Benicio Del Toro always insisted but was never overtly confirmed). Two, it seems Tivan sold Knowhere to the Guardians rather than try to rebuild his prized collection room. Where the Guardians got the money for such a massive real estate purchase is anyone’s guess. Apparently being three-time galaxy savers is good for the old bank account.

4. The GoBots May Exist in the MCU

5. Gamora Is Still Missing

... Excerpt from ‘Guardians Of The Galaxy Holiday Special‘: James Gunn On How Disney+ Show Bridges Franchise To Upcoming ‘Vol. 3’:

“I used the holiday special as an opportunity to introduce some elements into Guardians of the Galaxy, so I didn’t have to explain them at the beginning of Volume 3,” Gunn tells Deadline. “It was a trojan horse.

“I don’t have to talk about where they got [the new Guardians HQ] Nowhere from, or Cosmo being around Nowhere,” explains Gunn, who folded the threequel’s new sets into the special — i.e., Nowhere and the new spaceship, The Bowie. “I think you see what Nebula’s role is in the group now: She’s a leader to some degree.

“We see all these changes since we’ve seen them last,” adds the filmmaker.

The mega reveal here in the Holiday special: We learn that Chris Pratt’s Peter Quill and Pom Klementieff’s Mantis are brother and sister, fathered by Kurt Russell’s Ego.

... Excerpts from How Guardians of the Galaxy 3 is set up by the Holiday Special:

The Holiday Special takes place after we last saw the Guardians in Thor: Love and Thunder, and either since those events or before them, the team has been through some ch-ch-ch-ch-changes.

Nobody has left, but they've got a sparkly new massive ship called the Bowie, a new team member in the form of telekinetic talking dog Cosmo (voiced by Maria Bakalova) who we will instantly die for, and they now own – and seemingly run – Knowhere.

In Thor: Love and Thunder, the Guardians still had the Benatar, so they've clearly upgraded that in the time since that movie. Gunn has confirmed to a fan on Twitter though that the ship still exists, so it's not because it was destroyed.

As for Cosmo, it's unclear when the dog became part of the team other than it was after their adventures with Thor. They also might have been living in Knowhere at the time of Love and Thunder, but no specific timeline is given other than Nebula (Karen Gillan) confirming they bought the place from the Collector.

It's confirmed in the Holiday Special that Mantis and Peter are brother and sister as Mantis's father is also Ego (Kurt Russell). She had already told Drax the truth and she finally tells Peter at the end of the special, which he sweetly calls "the greatest Christmas gift I could ever get".

This revelation was originally part of Guardians of the Galaxy Vol 2, but the scene was cut from the final movie.

In the sequel, we were told that Mantis was found by Ego when she was just a larva and raised on his planet. However, there was a scene where one of Ego's love interests was Mantis's mother, confirming Mantis's familial connection to Peter.

Now that it's been restored as canon in the MCU, it strengthens the overall theme of Gunn's trilogy. Vol 1 was about finding your own family, which was further explored in Vol 2 where Peter chose his adopted family over his actual father.

If both movies have, in one way or another, been about Peter's mother and father, it stands to reason that a key theme of Vol 3 will be about Peter's relationship with his newly-discovered sister Mantis who is both his actual family and part of his adopted family.