Despite a dramatic opening week of public testimony on Capitol Hill, the House impeachment inquiry till now has been short on mystery or suspense. While witnesses last week painted a damning portrait of a president willing to abuse his office to secure investigations of his political rivals from a dependent foreign power, there were relatively few new revelations, and their stories were consistent with each other.

Wednesday’s testimony, however, moves onto unstable ground, which could make it the most important session yet. After a week of testimony from government officials who mainly experienced the Ukraine scandal as a force interfering with their normal foreign policy work, the public is due to hear from Gordon Sondland, the U.S. ambassador to the European Union, who reportedly sits at the center of that interference as one of the alleged principals in a presidential conspiracy to withhold $391 million in authorized security assistance from Ukraine in exchange for investigations of former Vice President Joe Biden and his son Hunter.

Critically, Sondland had multiple direct conversations with Trump, making him the first witness who could directly implicate the president in bribery. Further, he has changed his story repeatedly as it has proved inconsistent with the testimony of other witnesses.

President Donald Trump has distanced himself from Sondland—a million-dollar donor to his inauguration—going from describing him as a “great American” to saying, “I hardly know the gentleman” after the ambassador acknowledged that he had told a top Ukrainian official that security assistance was tied to investigations as part of a quid pro quo. Trump’s allies on the Hill have also begun to paint Sondland and the president’s attorney Rudy Giuliani as “loose cannons,” acting on their own without guidance from Trump.

Sondland faces legal peril, as House Democrats have suggested he may have perjured himself, and legal scholars are pointing to the potential that federal bribery statutes have been violated. All of this could motivate the Trump supporter to finally tell the full truth. Alternatively, he could continue to cite a faulty recollection of key events or even plead the Fifth, potentially placing himself in further legal jeopardy.

Five key episodes where his testimony diverged with that of other witnesses illustrate the legal dangers Sondland faces, why Wednesday’s testimony could directly implicate the president or Giuliani, and how the public could walk away from Wednesday’s hearing with a clearer sense that Trump committed bribery.

Episode 1: Advising Ambassador Marie Yovanovitch to “go big.”

Last week, former Ukraine Ambassador Marie Yovanovitch testified that Sondland encouraged her to “go big” in order to save her job and send out a tweet pledging political fealty to Trump. Yovanovitch testified this happened as she came under a Giuliani-orchestrated “smear campaign” that ultimately resulted in her removal from her post this past spring. Democrats argued that Yovanovitch’s ouster cleared the way for the eventual bribery scheme.

In his deposition to the Intelligence Committee, Sondland testified that he did “not recall” having a conversation encouraging Yovanovitch to tweet in support of Trump and would “probably” be surprised if anyone had testified to that effect. He will now have to clear up whether Yovanovitch’s testimony has refreshed his memory.

Episode 2: Telling Ukrainians at the White House to conduct investigations.

Former top White House Russia adviser Fiona Hill, who is due to testify Thursday, has also directly contradicted Sondland’s testimony on matters both big and small. First, Hill’s attorney put out a statement earlier this month saying that Sondland in his testimony had fabricated conversations the pair had over coffee. More importantly, Hill’s account of a crucial July 10 meeting at the White House differs from that of Sondland. Specifically, Hill says that Sondland told Ukrainian officials at an official meeting and then at a more informal debrief at the White House on July 10 that Zelensky would be granted a meeting with Trump if he went forward with investigations Trump was seeking. Hill is backed up in her version of events by fellow National Security Council official Lt. Col. Alexander Vindman, who testified publicly on Tuesday that Sondland raised specific investigations, including ones into Burisma and Biden, with the Ukranians.

Sondland testified that he didn’t remember telling the Ukrainians about these investigations. He also stated that if Hill “or any others harbored misgivings about the propriety of what we were doing, they never shared those misgivings with me, then or later.” Both Vindman and Hill say they confronted Sondland about the inappropriateness of the investigation request.

Episode 3: Trump asks about investigations during July 26 call from Kyiv.

One of the few new details to emerge out of last week’s hearings was the allegation that Trump spoke directly with Sondland about an “investigation” one day after his now-infamous July 25 phone call with Ukrainian President Volodymyr Zelensky in which he demanded an investigation of the Bidens. Over the weekend, an aide to Ukrainian chargé d’affaires Bill Taylor testified that he overheard a phone call between Sondland and Trump at a café in Kyiv on July 26 in which the president allegedly asked Sondland about Zelensky, “So, he’s gonna do the investigation?” During his testimony, Sondland did not mention that phone call. Instead, he made it seem as though his only call with Trump in that time frame was not relevant to the inquiry. “I do recall a brief discussion with President Trump before my visit to Kyiv,” Sondland testified. “The call was very short, nonsubstantive, and did not encompass any of the substance of the July 25, 2019, White House call with President Zelensky.” Holmes’ testimony seems to directly contradict this depiction. If Holmes’ version is true, this previously undisclosed detail would confirm that Trump was seeking to use Sondland as his emissary to push for his desired investigations.

Episode 4: Message delivered to Ukrainians that aid was contingent on investigations.

On Sept. 1, a delegation that included Vice President Mike Pence visited Warsaw, Poland, for a World War II commemoration. After Pence met with Zelensky and the security assistance was discussed, Sondland says he separately approached top Zelensky aide Andriy Yermak and told him that security assistance was contingent on announcing the investigations that had been sought in a series of text exchanges—probes that lined up with Trump’s request for an investigation of the Bidens. Sondland had to amend his initial testimony in order to acknowledge that this quid pro quo demand occurred. He only did so after Hill’s replacement on the National Security Council, Timothy Morrison, described it in his own testimony. Sondland further testified that “in the absence of any credible explanation for the suspension of aid, I presumed that the aid suspension had become linked to the proposed [investigations] statement.” That is contradicted, though, by Morrison’s testimony in the next incident.

Episode 5: Sondland tells Morrison that Trump conditioned aid on investigations.

Morrison testified that Sondland called him on Sept. 7 to tell him that he had just gotten off of the phone with the president and that he wanted to share what he had discussed with Trump. Morrison testified that on this call, Sondland “related the President told him there was no quid pro quo, but President Zelensky had to [announce the opening of the investigations] and he should want to do it.” Sondland did not mention this call in his initial testimony. In a follow-up affidavit to correct that previous testimony, Sondland would only state that he had “no specific recollection” of the phone call with Morrison. (Taylor backed up Morrison’s account, testifying that Morrison shared it with him contemporaneously.) Of course, if Morrison’s description of the conversation is accurate, it means that Sondland can directly implicate Trump in attempting to bribe Ukraine.