Sir Paul Stephenson's resignation last night was doubtless inevitable following the double revelation, within 48 hours, of his secret PR adviser and his subsidised stay at a five-star health spa. But the manner of his going at once underlined the extent of the crisis now gripping media, politics and policing in this country while also ensuring that the prime minister himself is now firmly in the spotlight over questions of judgment and his willingness to take responsibility for them.

Sir Paul's departure followed on from the disclosure on Friday that he had – unknown to politicians or the press – hired as his adviser on media strategy the former deputy editor of the News of the World at the time that the newsroom was something of a phone-hacking production line. Neil Wallis – who was a frequent dining companion of Sir Paul – was himself arrested on Friday morning in relation to phone-hacking inquiries by Sir Paul's own officers. Within 24 hours it was revealed that Sir Paul had enjoyed five week's of free accommodation at the luxury health spa, Champneys, while recovering from an illness. As luck would have it, Champneys' PR adviser was the selfsame Mr Wallis. Sir Paul's fate was sealed.

Sir Paul's long resignation statement protested his innocence in all respects. But one crucial passage effectively pointed the finger at Downing Street, drawing an comparison between Mr Cameron's hiring of Andy Coulson and his own recruitment of his deputy. The point was implicit, but widely understood: "I'll take responsibility: what about you?" And thus a crisis which, for a long time, was perceived as a relatively contained issue of journalistic ethics, started lapping at the door of the prime minister himself.

As Ed Miliband said yesterday, this saga is changing the very psyche of British politics. It was notable that the immediate reaction to the arrest of Rebekah Brooks at lunchtime was entirely sceptical. Was it a piece of chaff to distract attention from headlines about Sir Paul, or was it a conspiracy to give Ms Brooks an excuse for not giving evidence to MPs on the culture media and sport committee tomorrow? That detracts from the proceedings' significance, but not by much; the Murdoch dynasts, Rupert and James, will still be there. But the MPs should take care. They will have to work hard – and in a more disciplined way than last week – to prevent the hearing turning into a master class in media manipulation which allows the witnesses to avoid the real questions about which of them knew what, when. Those are the answers that must be ferreted out in order to expose the most important question: the real extent of complicity in criminal activities by News International's most senior figures. Those are the answers on which the Murdoch media empire hangs.

But the hearing down the corridor is in the domestic context almost as important. Sir Paul has much to tell the home affairs committee. He is certainly right to say that questions about the conduct of some very senior officers will hang damagingly over the Met for as long as the public inquiry takes.

Belatedly tackling the investigation with vigour, the Met has now made 10 arrests. The overwhelming majority are former NI people. These are threads in the web of influence that Rupert Murdoch has constructed. What it means is richly illustrated by Friday's publication of the prime minister's contacts with the media: in 15 months there have been 26 encounters with NI executives and editors, including at least four with Ms Brooks. It is just as richly illustrated by Neil Wallis. Ed Miliband was right to demand that media ownership be reconsidered. The dominance of News International is the gibbet on which hang the careers of two chief executives, two newspaper editors, some 200 journalists – and now the commissioner of the Metropolitan police.