The peer, Lord Hanningfield (aka Paul White), was imprisoned in 2011 over false claims for parliamentary expenses.
The paper has shown amazing patience because it obtained most of its detailed evidence about his activities back in July but had to wait six months, until Hanningfield's attendance claims appeared on the parliamentary website, before publication.
Its investigation was backed up by video. There is a timed film on its website showing Hanningfield arriving at Westminster tube station and, 30 minutes later, after his visit to the Lords, returning to the station to travel back to his Essex home.
Apparently, the peer was not guilty of breaking any rules. But that's the point, of course. The Mirror is arguing that the rules are totally inadequate, as its editorial makes clear:
"The case for sweeping reform is unanswerable. This is not just about blowing away a few cobwebs. This is about creating a parliament which works for the British people, calling time on a corrupt system which sees people working and paying their taxes to sustain privileged members of a luxurious club."
It accuses Hanningfield, of "playing the system" and calls for a series of reforms. First, imprisoned peers should bot be allowed to return to parliament.
Second, there must be checks to ensure peers "put in a fair day's work" to earn their £300 allowance. However, it is already there case that the daily allowance is available only to peers "who certify that they have carried out appropriate parliamentary work".
Can Hanningfield prove that to be the case? As part of his justification, he said he was one of maybe 50 noble lords who did much the same.
No wonder the Mirror's associate editor, Kevin Maguire, calls in an accompanying polemic for "unelected peers" to be binned. His is a full-frontal assault on the institution and concludes with a call, not for reform, but abolition.
Comment: This is a modern Mirror harking back to its hallowed past as the champion of the people. It is a mix of reporting initiative, excellent projection, topical front page headline and class-based comment. The late Hugh Cudlipp would have praised today's issue to the skies.
Sure, it's only one day. But the Mirror's Mandela coverage was good too. There is more than a sense of the paper returning to its editorial roots, making a serious attempt to be more serious (with plenty of entertainment and trivia in between).
So well done to the editor, Lloyd Embley, for living up to his promise to refreshen his paper. Mind you, I still find the layouts hard to take...