expression, noun
circumstances; how things happened
A formal, somewhat literary expression used to describe the sequence or details of events, often in explanations or reports.
事の次第を詳しく説明してください。
Please explain the circumstances in detail.
事の次第はこうです。
Here is how things happened.
経緯 also means 'circumstances' or 'details', but is more common in modern formal contexts, while 事の次第 has a slightly more literary or old-fashioned feel.
A fixed phrase combining 事 (thing, matter) and 次第 (order, course). The exact historical derivation is uncertain, but it has been used since classical Japanese to mean 'the way things went'.