Nick Read, the Post Office's outgoing chief executive, will give three days of testimony from Wednesday, amounting to a grilling of about 16 hours.
Read, who was brought in to replace Paula Vennells in 2019 and "right the wrongs of the past", announced in the summer that he was stepping back from running the company to focus on preparing and proving that "nothing like this could happen again".
The executive, who has given himself 88 days to prepare for the appearance, has been heavily criticised by witnesses and has become embroiled in his own reputational crisis.
Here are some of the issues the judge-led inquiry is likely to ask him to explain.
'Obsession' with pay and bonuses
Henry Staunton, the former chair of the Post Office, twice asked the government for Read's pay to be doubled, and has told the inquiry that Read had threatened to leave over the issue.
Staunton, who was sacked in January by then business secretary Kemi Badenoch, said he only put forward the requests because he knew they would be refused.
"It was astonishing," Staunton told the inquiry last month. "It was taking up a disproportionate amount of my time. If I was a subpostmaster I would have been horrified with my remuneration going up 1% or 2%, costs going up more, many losing money - to see discussion of moving [Read's] pay to £1.1m just for hitting target, let alone the amount he would have got for good performance."
Jane Davies, the former people officer at the Post Office, bluntly called it an "obsession" while Amanda Burton, the chair of the Post Office's remuneration committee, said she was surprised at how often the topic of Read wanting a pay increase was raised.
The 'untouchables'
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