After a brief flashback to 1980 where we get to see Peter Quill's mother, Meredith (Laura Haddock) having a romantic encounter with a man who appears to be Peter's mysterious father, GotG2 picks things up at what seems like a few months after the end of the first film, with the Guardians - Peter Quill (Chris Pratt), Gamora (Zoe Saldana), Drax (Dave Bautista), Rocket (voiced by Bradley Cooper) and now-baby Groot (voiced by Vin Diesel) getting ready to protect some highly valuable Anulax batteries from a giant marauding space beast. We soon learn that they were hired for the job by a high priestess named Ayesha (Elizabeth Debicki), the supreme leader of a gold-skinned and rather arrogant race called the Sovereigns, who apparently don't like getting their hands dirty or putting themselves personally at risk. As part of their payment, the Sovereigns hand over Gamora's step-sister Nebula (Karen Gillan) whom they happen to have captive. But things quickly go south when Rocket decides to help himself to some of the Sovereigns' precious batteries because "Hey, they were really easy to steal!" The subsequent pursuit by the outraged Sovereigns sets everything in motion and leads to the introduction of some new characters - Ego (Kurt Russell), a Celestial who claims to be Peter's father, and Mantis (Pom Klementieff), an eager-to-fit-in but rather naive empath who serves Ego - along with the reappearance of Yondu Udonta (Michael Rooker), leader of the band of Ravagers who abducted and then essentially raised Peter from when he was a boy.
One note: while GotG2 is a lot of fun to watch, I felt it didn't measure up to the first film in terms of smoothness of flow. The best scenes are when the Guardians are in action and/or when they're interacting with each other, and there are plenty of both throughout the film. But that said, a lot of the purpose of GotG2 is to provide both background on the characters and, to a lesser degree, some setup for what will come next, and in doing so, there are a number of places in the film where the pace stumbles a bit due to one or more characters having to engage in what is generally called lumpy exposition. This is why I gave the film 4 stars instead of 5. It's the only reason, mind you, and the film is still pretty awesome overall, but at the same time these exposition lumps, however necessary they may be, do slow things down a bit.
But there is a lot of heart in this film, just as there was in the first, and GotG2 does succeed in deepening our understanding of - and affection for - the Guardians as a group and as individuals. The most apt exchange defining this is when Nebula fumes to Gamora "All you do is yell at each other. You're not friends!" To which Drax responds "No. We are family."
Highly recommended to any fan of the MCU franchise and to anyone who enjoys good, fun space opera with a healthy dose of heart.
PS - (1) There are a lot of cameos and Easter eggs in this film. Too many to list here. I suggest googling for them so you'll know what to watch for in the movie.
(2) There are 5 credits/after-credits scenes at the end of GotG2.
(3) The film also has a recurring "crazy credits" going on so keep an eye on the credits themselves as they scroll by.