The publication of the Home Affairs Select Committee's report into antisemitism is a defining moment.
Jeremy Corbyn's failures have been detailed and exposed for all to see. With stark clarity, the select committee has ensured that the excuses have dried up. Until Mr Corbyn became Labour leader, it was inconceivable that the leader of any serious political party could ever have been condemned by a select committee for creating "a 'safe space' for those with vile attitudes towards Jewish people".
Since his assumption of the post, however, it has no longer merely been conceivable - it has been long overdue that Mr Corbyn and his followers should be taken to task for their consistent unwillingness to deal properly with antisemitism in their ranks.
The select commitee is to be commended for its rigour and honesty - which stands in contrast to the whitewash produced by Baroness Chakrabarti, who is rightly pilloried by the committee for her "compromised" report.
Nothing better illustrates the problem with the Labour leader than his response to the select committee's report. Instead of using it as an opportunity to improve his understanding of the issue and develop Labour's mechanisms for dealing with it, he attacked the MPs on the commitee for "politicising antisemitism".
Self-awareness appears not to be one of Mr Corbyn's most prominent traits; no senior politician since the Second World War has done more to politicise antisemitism than the Labour leader himself, whose behaviour has brought the issue back from the fringes.
It feels as if we have now reached the end of Act I of this appalling drama. No one yet knows what further twists will emerge in Act II.