Explaining the ties between Ukraine and Joe Biden's son

President Trump today dismissed allegations that he asked Ukraine’s president to dig up dirt on former Vice President Joe Biden. 

Biden hit back while he was on the campaign trail in Iowa this weekend, lambasting a reporter who asked him about his son Hunter Biden’s overseas business dealings. "Trump’s doing this because he knows I’ll beat him like a drum,” said Biden. “And he’s using the abuse of power and every element of the presidency to try to do something to smear me. Everybody’s looked at this, everybody’s looked at it, and said there’s nothing there. Ask the right questions.” 

It’s the latest back-and-forth in the big whistleblower story that’s been developing over the past week. We get an explainer of what's going on, and look at the legal questions involved.