Today is the first day in which the House Intelligence Committee’s impeachment investigation of President Trump will be held in public—as called for in the resolution adopted two weeks ago by the full House of Representatives. Here are a few things to look out for:

1. The big question for Democrats to answer is not “What happened?” but “Why does what happened even matter?”

Although new details could emerge in the impeachment hearings, the basic facts about what happened—the quid pro quo/extortion of the Ukrainians by withholding $391 million in foreign aid unless President Zelensky announced a criminal investigation into Joe Biden and his son, as well as the shadow foreign policy towards Ukraine that was orchestrated by Rudy Giuliani—are already public knowledge. These facts have been widely discussed and dissected. They remain largely unrebutted on the merits by President Trump, his staffers, and his congressional allies.

The president has three options for his defense. Option one is distraction—that is, he could argue that the House procedures are unfair (an argument that is much weaker now that the hearings are open and transparent) or that the unsubstantiated wrongdoing by the Bidens is the real problem here.

Option two is scapegoating. The president and his congressional supporters could demand that the whistleblower testify (as if that individual’s point of view is relevant now that witnesses with firsthand knowledge have gone on record), and then impugn the whistleblower’s credibility.

Option three is to argue that what happened isn’t impeachable in the first place. Acting White House Chief of Staff Mick Mulvaney made this argument in his now-infamous press conference: It happened, sure, but “get over it.” Former U.N. ambassador Nikki Haley also argued this week that none of this matters constitutionally.

This last defense is the biggie: It will likely be central to the ultimate fate of the impeachment process in the House.

2. Democrats need to tell a persuasive and gripping story. They are starting the story with Bill Taylor. Why does Bill Taylor matter?

William B. Taylor is a career diplomat who served under Republican and Democratic administrations. He is currently the U.S. chargé d’affaires for Ukraine—that is, the acting ambassador—and previously served as the ambassador to Ukraine from 2006 to 2009.

Given that background, Taylor is perfectly suited to explain why Giuliani’s “irregular” diplomatic channel was a problem. The country has sought to preserve its independence ever since it declared it in 1991. For the Western democracies, Ukraine is a bulwark against Russian expansionism. (“Ukraine” literally means “frontier.”) Giuliani’s influence worked against the objective of fostering democracy in Europe, including in Ukraine—a longstanding U.S. policy goal that is obviously of great benefit to the United States.

It is important to keep in mind here that there are actually two narratives at play in this part of the impeachment inquiry.

The first—which most people focus on—is the quid pro quo/extortion narrative: Did Trump withhold Ukrainian military aid as leverage to get Zelensky to investigate a political rival and his son? Note that bribery is a cousin of extortion, and it’s an express basis for impeachment in the Constitution.

The second—and equally important—narrative involves the question of whether Trump handed off Ukrainian foreign policy and a chunk of the State Department to his personal lawyer, private citizen Rudy Giuliani, whose loyalty is to Trump personally—not to the Constitution, the rule of law, or the American people.

Taylor can talk about both narratives, but he has no firsthand knowledge of the former.

3. Why does George Kent matter?

George Kent is also scheduled to testify today. Like Taylor, Kent is a high-level career official in the State Department who oversees U.S. policy over Ukraine. And, also like Taylor, Kent can testify to the second narrative—the shadow foreign policy under Rudy Giuliani. He can also talk to the accuracy of the president’s claim that the White House was seeking to root out corruption in Ukraine, not to secure an advantage in the 2020 election by persuading the Ukrainians to disparage the Bidens.

Like Taylor, Kent has no firsthand knowledge of the first narrative.

Kent will also set the stage for former Ambassador Marie Yovanovitch, who is supposed to testify on Friday. She is expected to explain that she was ousted not because she wasn’t doing her job for the United States but because she wasn’t doing her job according to Giuliani’s objectives.

4. What other potential witnesses will really matter, and why?

In the days ahead, much will hinge on witnesses who can speak to both narratives—who have both firsthand knowledge of the quid pro quo/extortion by Trump as well as of the president’s policy directives to Giuliani.

Put another way: If you actually saw rain coming down, that would be more persuasive information about the weather than if someone told you that it rained, or if you saw that the grass was wet in the morning and just inferred that it rained overnight.

We don’t yet know all the witnesses who will be called to testify, but Lt. Col. Alexander Vindman is important in this respect because he was actually on the July 25 call with Zelensky. One defense that Trump continues to make is that there was no quid pro quo on that “perfect” call. Vindman has firsthand knowledge of what was actually said.

Gordon Sondland, Mick Mulvaney, John Bolton, and Rudy Giuliani are also important potential witnesses here. If these men are subpoenaed to testify—and if they actually comply with the subpoenas, which is a big “if”—their testimony could be explosive. They were close to the president and thus could have firsthand knowledge of what Trump directed. Remember that Sondland changed his closed-door deposition testimony given under oath to state that he actually had communicated to the Ukrainians a demand for an investigation in exchange for military aid.

This is not a close case on the facts, friends. Sadly, it might turn out to be a hard case on the question of “who cares.” The right answer is that we all should.