As the Marvel canon (the MCU, or "Marvel Cinematic Universe") increases in film count and number of characters, their version of Earth diverges farther and farther from the one we live in - all leading up to "Infinity War." Black Panther was introduced in possibly my least favourite MCU movie, Captain America: Civil War. Now we're expected to accept that the country Black Panther comes from, Wakanda, has had massively advanced technology for a century or more, but has still succeeded in pretending to be a backwards farming country in the middle of Africa for that entire time and NO ONE HAS NOTICED. Except the arms dealer Klaue (Andy Serkis, who we first met in Avengers: Age of Ultron). He's back - and pissed off, because they took his arm. And they're pissed off because he detonated a bomb to cover his escape that killed dozens (or hundreds - precision isn't a requirement in the MCU).
If you can accept the MCU baseline (and obviously millions of moviegoers do), then this is an enjoyable coming-of-age - or perhaps coming-of-kinghood - tale. Chadwick Boseman returns as the Black Panther, and with his father dead (see "Civil War"), he goes through the ceremony to become King - not an easy process. But much worse is ahead, with an upset Wakandan-American with royal bloodlines headed home.
I can't tell you much more without starting to give stuff away. It's fun, it's charming, there's bucket-loads of patented MCU action (invariably free of blood despite several deaths by skewering, so they can retain their family-friendly ratings at the theatres). Not my favourite Marvel film, but far from the worst. I would have preferred to see it on DVD from the library rather than giving my money to Marvel at the theatre, but I had fun so I'm not significantly upset about it.
I was also surprised that most of the people in the theatre left before the end of the credits. Don't they know this is Marvel? There is, inevitably, not only a mid-credits scene (Wakanda speaks to the U.N.) and a post-credits scene (Bucky Barnes on the mend).