Bill Taylor laid out the most compelling evidence to date of a quid pro quo involving President Trump and Ukraine. In doing so, he became the first U.S. official to describe an explicit request involving official U.S. government concessions potentially being exchanged for politically advantageous investigations for Trump.

The acting top U.S. diplomat in Ukraine said not just that this setup involved both military aid and Ukraine’s desired meeting between its president and Trump, but also that it involved requested investigations focused on the Bidens and a conspiracy theory about Ukraine’s role in interference in the 2016 U.S. election.

But Taylor’s version won’t be the final word. His testimony both provides important clues about what future witnesses could say and indicates that Ambassador to the European Union Gordon Sondland, who testified last week, has a lot of explaining to do.

So what do we need to find out next?

The first big question involves an aide who figures prominently in Taylor’s narrative: Tim Morrison, the head of the White House National Security Council’s Eurasia desk. In arguably the most significant disclosure by Taylor, Morrison allegedly told him that Sondland had relayed an explicit quid pro quo to a top aide to Ukrainian President Volodymyr Zelensky, Andriy Yermak.

"During this same phone call I had with Mr. Morrison, he went on to describe a conversation Ambassador Sondland had with Mr. Yermak at [a meeting in] Warsaw,” Taylor said in his opening statement. “Ambassador Sondland told Mr. Yermak that the security assistance money would not come until President Zelenskyy committed to pursue the Burisma investigation.”

(Zelenskyy, the spelling used by Taylor in his written remarks, is the preferred spelling in Ukraine.)

Taylor also describes Sondland’s telling him personally that “‘everything’ was dependent on such an announcement [of investigations], including security assistance.” But Morrison’s corroboration appears particularly important. He would be another firsthand witness indicating Sondland’s directly passing along the quid pro quo to Ukraine. Thus far, we have Sen. Ron Johnson (R-Wis.) pointing in that direction, as well as acting White House chief of staff Mick Mulvaney’s later-rescinded admission.

Another key element is what Sondland might say from here on out. As I noted in my takeaways post, there appear to be major discrepancies between what he said in his testimony last week and what Taylor said Tuesday.

A key example: Sondland said, “I recall no discussions with any State Department or White House official about former vice president Biden or his son, nor do I recall taking part in any effort to encourage an investigation into the Bidens." Compare that to Taylor’s hearing about Morrison’s version of events, and it’s difficult to square them.

Perhaps Sondland’s defense will be that he pushed for the Burisma investigation and not one specifically into the Bidens. That would be quite the rhetorical trick, but it would fit neatly into his testimony, in which he claimed he “did not understand, until much later, that Mr. [Rudolph W.] Giuliani’s agenda might have also included an effort to prompt the Ukrainians to investigate Vice President Biden or his son ...” He said that, as of August, “I did not know until more recent press reports that Hunter Biden was on the board of Burisma.”

I wrote about how difficult that was to swallow, given that Hunter Biden’s work for Burisma and Giuliani’s efforts to pursue that investigation in Ukraine had been in the news for months. But now we may know why Sondland was attempting to walk such a fine line. Perhaps he knew what Taylor would say, and his new defense — however implausible — would be that he didn’t expressly or implicitly call for investigating the Bidens, because he didn’t know that was the same as Burisma.

Once we go beyond Taylor’s testimony, there is the matter of the White House transcript of that Trump-Zelensky call. There continue to be unanswered questions about whether it was the full picture, with ellipses coming at key junctures when Trump was talking about the investigations mentioned above. We knew that it was a rough transcript based upon notes taken by those listening in on the call, but there are some indications and suggestions that there might be a word-for-word transcript somewhere.

And one last timely and potentially major matter is what happens next with Giuliani. There have been mixed messages about whether he is under investigation and if so, for what, after two business associates of his, Lev Parnas and Igor Fruman, were indicted recently. Republicans, including Trump, have distanced themselves from Giuliani, perhaps worried about how brazen he might have been in this whole time frame. We’ve already seen what happened when one of Trump’s personal lawyers got into trouble (see: Cohen, Michael); it’s not difficult to think what might happen if Giuliani is truly in hot water.