NEW YORK (FORTUNE Magazine) - At about 4:30 p.m. on Wednesday, prosecutor Kathy Ruemmler finished questioning Ben Glisan, who was Enron's treasurer.

It's a moment worth marking, because Glisan is the government's last big witness. (That said, you never know who might appear in the rebuttal phase of the case -- namely, former chief accounting officer Rick Causey).

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Glisan, 40, who looks a tiny bit like a young Spock, was expected to be a powerful witness for the prosecution. Once a big Texan, he is now thin, even gaunt, from the two-plus years he has already served of a five-year prison sentence.

Because Glisan, unlike other witnesses, went straight to jail after he pled guilty in the fall of 2003, the defense will have difficulty using the same argument that it has used with other witnesses -- which is that they're simply saying whatever the government wants to hear in a bid to reduce their sentences. That, along with his obvious intelligence, his intimate knowledge of accounting, and his senior role at Enron, makes Glisan a very dangerous witness.

And Glisan delivered. He doesn't mince words. Often, when Ruemmler would ask him if a statement Lay had made was true, he would say flatly, "No."

"Did you discuss your concerns with Mr. Lay?" was another question Ruemmler frequently asked.

"Yes," replied Glisan.

Glisan, an inveterate note-taker back in the day, provided ammunition for the government's contention that Fastow had verbal guarantees from Enron on the profits that his LJM fund would make in its deals with Enron. Glisan says that he often served as a mediator in negotiations between Fastow (representing LJM) and Causey (representing Enron), and indeed, his notes reflect various bargains between the two.

"ROC [return of capital] plus 100,000 plus 15% IRR [internal rate of return]," read Glisan's notes on the Nigerian barge deal.

Glisan -- whose guilty plea arose from the infamous Raptor deals -- also discussed why he believed the structure he created failed to meet accounting rules (The problem: LJM would get an upfront return "of and on" its principal.)

"Mr. Skilling understood it and liked it," he said of the Raptors.

And so did Ken Lay, Glisan testified. When he presented it to the finance committee, Glisan recalled, "Mr. Lay giggled."

"He giggled?" asked Ruemmler.

"In delight," responded Glisan. (Glisan was appointed as Enron's treasurer on the very day that the Raptors were approved by the board.)

Indeed, Glisan's most powerful testimony involved Ken Lay. Repeatedly, he said that Lay knew full well just how troubled Enron's international assets were (a $500 million loss on Dabhol was a "best case scenario," he said), just what bad shape the water business was in ("it was roughly $1 billion under water," he said), and more.

At a gloomy retreat for top executives at the Woodlands in the fall of 2001, Glisan quoted another executive, John Lavorato, as saying he was "glad he didn't have a gun or he would shoot himself."

All the while, of course, Lay was telling investors and employees that Enron was in great shape. As Glisan's testimony made clear, the defense's theory that all of Enron's problems were caused by Fastow and his rogue group, along with run-amok shortsellers and journalists, is a deeply problematic one.

"We've lost the benefit of the doubt," Glisan jotted on October 22, 2001. "Do they earn money or manufacture it?" was a question he got from bankers.

Over and over again, prosecutor Ruemmler asked Glisan if the growing concerns on the part of bankers and rating agencies were justified.

"I think they were rightly concerned," responded Glisan, who said at one point, "We were an economically weak and strained company that had systematically hidden that weakness for a long time."

Glisan also testified that he told Lay in the fall of 2001, just after Fastow was fired, that he too had made money from Enron's partnerships -- he had netted $1 million from the Southhampton partnership off an investment of just under $6000. After telling Lay, Glisan said, he had offered to resign.

Lay's response? "... [That] I shouldn't and that he thought my participation was OK and that the last thing I should do was resign."

This is particularly interesting in light of the defense's contention that Enron's bankruptcy was the result of the "stench" of hidden wrongdoing on the part of Fastow, Glisan, and Kopper. If their behavior was so abhorrent, so out of keeping with Enron's culture, why didn't Lay fire Glisan on the spot?

In the courtroom, Lay -- who has appeared relaxed and even smiling through months of prosecution testimony -- seemed more agitated than at any time to date in the trial. When Glisan testified that he told Lay on October 23 that "bankruptcy was inevitable," Lay shook his head at the defense table. At other times, he whispered frantically to his lawyers.

Expect defense attorneys to go after Glisan hard starting today -- with Lay's attorneys, for a change, spending the most time on the attack. One line of questioning is sure to revolve around the specifics of his agreement to testify.

Although Glisan says "I have no such expectation" about getting a break on his jail sentence, he has gotten to spend time with his family on the furloughs he's been granted to talk to the government. Defense attorneys seemed to scent something in that, and even if it was just a whiff, you can be sure they'll try to turn it into a stink.

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