Trump Ukraine call: What's this story all about?

https://www.bbc.com/news/world-us-canada-49800181

There's a new political controversy in the US - involving Donald Trump, foreign nationals, questions about legal and ethical behaviour, and allegations against a political rival.

This feels a bit like déjà vu from 2016, with Russia, then-candidate Trump and Hillary Clinton, but it's a new country (Ukraine) and a new cast of characters (Joe Biden and his son Hunter).

Mr Trump is still right smack in the middle, of course.

The story can be difficult to follow, so here are some answers to the most pressing questions.