A mystery whistleblower, a former comedian and the president of the United States. These are some of the main players in a story that is becoming ever more complex - and could see the president being impeached. 1) Donald Trump Who is he? The president of the United States of America. What's his role? Without him, there would be no story. Here's what we know about his involvement: - Mr Trump himself has acknowledged that he personally blocked nearly $400m in military aid to Ukraine. - At about the same time, he spoke by phone with Ukraine's new president. - In the call, Mr Trump pushed Ukraine's president to investigate his leading domestic political rival, Joe Biden. - A complaint by a whistleblower in the intelligence community, who spoke with White House sources about the call, alleges Mr Trump used "the power of his office to solicit interference from a foreign country in the US 2020 election" Mr Trump says the investigation is part of a "witch-hunt" against him, and he denies that military aid was withheld in order to put pressure on Ukraine. He has also demanded to know who gave information to the whistleblower, saying the source was "close to a spy". 2) Volodymyr Zelensky Who is he? Like Mr Trump, he's a former TV star. A one-time comedian with more than 8.5 million Instagram followers (who now see fewer of his exercise videos, but more photos of him meeting world leaders), he was elected president of Ukraine in a landslide win in April. What's his role? The man on the other end of that 25 July phone call. As our man in Kiev explains, the contents of the call proved to be a bit embarrassing for Mr Zelensky when they were released. Not only were there a number of attempts to flatter Mr Trump, but it appeared that Mr Zelensky was willing to open an investigation into Mr Biden's son, at Mr Trump's request. The big question - and one US investigators may be keen to ask Mr Zelensky and his associates - is whether the call (the two presidents' first detailed conversation since the Ukraine election) took place with a pre-condition that the Biden case be discussed. 3) Joe Biden Who is he? The vice-president under Barack Obama's presidency for eight years, he is now the frontrunner for the Democratic nomination for the 2020 presidential election. What's his role? It's the fact that Mr Biden is leading polls to be the Democratic candidate that is the most relevant detail here - if chosen, he would be the man facing off against Mr Trump for the presidency in November 2020. Most polls suggest Mr Biden would win in this scenario. In brief: Mr Trump alleges Mr Biden abused his power while in office. He suggests Mr Biden pressured Ukraine to back away from a criminal investigation that could implicate his son, Hunter (more on him later). The issue is that if Mr Biden was just a retired member of society right now, Mr Trump's request to Ukraine would be of little consequence. But because Mr Biden is (for now, at least) his biggest rival for the presidency, it opens Mr Trump up to claims he was working with a foreign power to influence the election. "This isn't about me," Mr Biden said on 26 September, "it's a tactic that's used by this president to try to hijack an election so we do not focus on the issues that matter in our lives." 4) Hunter Biden Who is he? Mr Biden's 49-year-old son, who has worked as a lawyer and lobbyist. What's his role? His part in this convoluted saga relates to his position on the board of Burisma, a Ukrainian gas company, which he held for almost five years from 2014. While Joe Biden was vice-president, Ukraine's most senior prosecutor was investigating the company, but was then dismissed. Mr Trump and his allies have suggested that Joe Biden encouraged the prosecutor to be fired. But there has been no evidence of wrongdoing by Hunter Biden - and the Ukrainian prosecutor who replaced the one who was fired (and continued his investigation into Burisma) told the BBC there was no reason for Ukraine to investigate the Bidens and that any corruption with Burisma happened before Hunter Biden joined the board. 5) The whistleblowers Who are they? Exactly. What is their role? At least two whistleblowers have come forward. The first wrote to the chairmen of senate committees on 12 August expressing concern over Mr Trump's phone call with the Ukrainian president on 25 July. They also alleged that the White House acted to "lock down" all details of the phone call between Mr Trump and Mr Zelensky, and that the call transcript was not stored in the usual computer system. All we know about the whistleblower #1 is that they are a US intelligence officer - or at least were, when they wrote their letter. They specify in their letter to Senate officials that they do not work in the White House, but it is clear they are well-connected to people who do. US media have reported they are a CIA officer who once worked at the White House. Their lawyers then said a second whistleblower from the intelligence community had come forward (reportedly with direct knowledge of the allegations related to the Trump-Zelensky call). Lawyers then said "multiple" whistleblowers had emerged. 6) Nancy Pelosi Who is she? The woman who, as the Speaker of the House of Representatives, could hold the key to Donald Trump's future in her hands. What's her role? Ever since Democrats retook control of the House last November, Ms Pelosi has resisted calls by some within her party to start impeachment proceedings against Mr Trump. That changed on 24 September, when she announced the opening of a formal impeachment inquiry, saying the president "must be held accountable". Why has Nancy Pelosi made her move now?Impeachment isn't the same as a criminal conviction, and it doesn't necessarily mean Mr Trump would be removed from office. If the inquiry moves forward, the Democratic-controlled House of Representatives will vote on any charges. If Democrats were to remain united, the measure would be carried - and Mr Trump would become the third president in US history to be impeached. But the proceedings would be expected to stall in the Senate, where the president's Republican party holds enough seats to prevent him from being removed from office by a two-thirds majority. 7) Rudy Giuliani Who is he? The former mayor of New York and, most relevantly to this story, President Trump's personal lawyer. What's his role? Mr Giuliani, one of Mr Trump's most vocal cheerleaders, has been central in pushing the suggestion that the Bidens were involved in wrongdoing in Ukraine. He has been speaking to Ukraine's prosecutors - past and present - about the case since late 2018. In an interview with the Atlantic, Mr Giuliani said he was simply "straightening out government" by calling for investigations into the Bidens. "It is impossible that the whistle-blower is a hero and I'm not," he said. "And I will be the hero! These morons - when this is over, I will be the hero." 8) Viktor Shokin Who is he? The prosecutor fired by Ukraine's former government. What's his role? Joe Biden has been quite open about the role he played in getting Viktor Shokin dismissed, saying there were concerns he was not doing enough against corruption. Mr Biden even said he threatened to withhold $1bn in aid to Ukraine unless Mr Shokin (the man who, at the time, was investigating his son's company) was fired. In January 2018, he recounted a conversation with Ukraine's government. "I said, you're not getting the billion," Mr Biden said. "I'm going to be leaving here in, I think it was about six hours. I looked at them and said: 'I'm leaving in six hours. If the prosecutor is not fired, you're not getting the money.'... He got fired." Mr Biden was not the only political figure to demand Mr Shokin be dismissed, however, and other Western nations and the International Monetary Fund had criticised the prosecutor's inaction on corruption. 9) Kurt Volker Who is he? Until recently, he was the US government's special envoy to Ukraine. What's his role? He was the middle man between the White House and Zelensky's government, and appears to have helped urge the Ukrainians to investigate the Bidens. After Mr Volker gave evidence to lawyers in Congress in early October, the details of text messages (some of which involved Mr Volker) were released. In one, he indicates a visit by Mr Zelensky to Washington would happen only if he agreed to an investigation into the Bidens. "Heard from White House - assuming President Z convinces Trump he will investigate / "get to the bottom of what happened" in 2016, we will nail down date for visit to Washington," the message said. Another message shows he suggested what Mr Zelensky should say in a public statement. 10) Gordon Sondland Who is he? The US ambassador to the European Union and a close ally of President Trump. What's his role? Mr Sondland - a hotelier who donated $1 million to the president's inauguration committee - was appointed EU ambassador in 2018. He first came under scrutiny after text messages surfaced that showed him discussing an effort to pressure Ukraine. Mr Sondland gave closed-door testimony to Congress in which he denied that military aid was withheld from Ukraine for political reasons. But, in a major reversal, he later revised his testimony and said he had told a Ukrainian official that the aid "likely" hinged on the country opening a political inquiry. It was a significant revelation given that Mr Trump has denied using the aid as a bargaining chip. 11) William 'Bill' Taylor Who is he? A senior diplomat and the acting US ambassador to Ukraine. What's his role? He's delivered some of the most explosive testimony in this inquiry so far. Mr Taylor told a private hearing that it was his "clear understanding" that the president had withheld military aid because he wanted Ukraine to investigate the Bidens. He also said that Mr Trump's personal lawyer, Rudy Giuliani, was behind the drive to get Ukraine to announce an investigation. Mr Taylor will be the first official to testify in public, televised, hearings in Congress.