Smoking guns come in all sizes and shapes at Westminster.
On Tuesday, we will learn if Sir Olly Robbins has one, if it’s loaded and whether it will trigger the end of Sir Keir Starmer’s premiership.
Just a week ago, questions about Sir Keir being toppled within months were ebbing away.
The Prime Minister had stood up to Donald Trump over his Iran war, a move backed by Labour MPs, party members and in tune with the vast majority of Britons.
Kemi Badenoch and Nigel Farage had been caught politically flat-footed after initially being far more bellicose.
The threat of a coup against Sir Keir after the May local elections, when Labour is facing a hammering including in London, was receding.
But this brief lull in Sir Keir’s seemingly endless woes was blown away by the revelation that Lord Mandelson was appointed by the PM as Britain’s ambassador to the US despite having failed to be given the all-clear in security vetting.
Now, the PM is again fighting for his political life, with calls for him to resign over the latest episode of the Mandelson scandal.
He has told of his fury at being kept in the dark by the Foreign Office (FCDO) that the Labour peer was given security clearance despite the latter’s vetting process failure.
The PM swiftly fired the FCDO’s top civil servant Sir Olly to outcries that he was being made a “scapegoat” and being “thrown under the bus” by No10.
Key to the row appears to be the fact that Sir Keir had announced Lord Mandelson’s appointment to America before he had been fully vetted.
Sir Olly was put in a position where the Prime Minister had “stacked the deck” in favour of the peer being given the all-clear to take up the post, according to one source.
So was the Foreign Office mandarin simply and quietly fulfilling the PM’s wishes?
Senior MPs strongly doubt that Sir Olly would have acted alone in taking such a crucial and controversial decision, and suspect there must have been political pressure applied on him.
The former FCDO chief is due to give his account of the storm when he appears before the Commons Foreign Affairs Committee on Tuesday.
So, if he does have a smoking gun, of any political pressure having been applied on him, how damaging will it be for the PM?
One of the most pertinent examples for this case may possibly have involved Peter Mandelson himself.
Not his first sacking from the Cabinet as Trade and Industry Secretary in 1998 over an undisclosed £373,000 home loan from then Paymaster General Geoffrey Robinson.
Nor his third sacking, as British ambassador to the US, over the revelations in the Epstein files over his links to the paedophile financier.
But the one sandwiched in between them, the Hinduja passport affair.
Tony Blair sacked then Mr Mandelson as Northern Ireland Secretary in 2001 over a British passport application by Indian billionaire Srichand Hinduja.
After days of swirling controversy, No10 admitted Mr Mandelson had called Immigration Minister Mike O'Brien to discuss the application.
Mr O’Brien remembered the call, Mr Mandelson questioned whether it had taken place, an inquiry found it was “likely” they had spoken.
The Cabinet minister maintained there was "no wink or nudge" involved in the "innocent inquiry" he, or his office, made on Mr Hinduja's behalf.
But the reported phone call was enough for Mr Blair to force him to resign.
An inquiry later cleared Mr Mandelson of wrongdoing but the case highlights how a phone call, no matter how seemingly innocent, can be seen, rightly or wrongly, as applying political pressure.
So on Tuesday, Westminster will be gripped to discover if there were any similar phone calls, messages or other pressure from No10 put on Sir Olly.
If he just felt he had to do the PM’s bidding, given that the diplomatic appointment had already been announced, that is one thing, and it may not be hugely damaging to Sir Keir.
But if there are any signs of political pressure, the PM’s grip on power will be further weakened.
Prime Ministers. though, hold the levers of power so they are in a stronger position than Cabinet ministers caught up in storms who can be sacked if they are judged to have erred badly.
Boris Johnson withstood a barrage of revelations over the Partygate scandal and even a damning inquiry finding that he had misled Parliament.
It was his handling of the Chris Pincher groping row which finally brought him down when his Cabinet, led by Rishi Sunak and Sajid Javid, finally lost patience with him.
Sir Keir’s Cabinet rallied around him after Scottish Labour leader Anas Sarwar called for him to go and the party has yet to resolve its key leadership dilemma, if not Keir, who?
Former Labour Deputy Leader Angela Rayner appears eager to enter a leadership race but she still has a tax affairs row hanging over her and some MPs on the Labour Right have doubts about her as a possible PM.
Health Secretary Wes Streeting had strong ties with Lord Mandelson and may struggle to win support on the Labour Left.
Net Zero Secretary Ed Miliband may be the compromise candidate but the nation has already rejected him once.
Greater Manchester Mayor Andy Burnham would be in a strong position but Labour chiefs, including Sir Keir, blocked his attempt to get back into Parliament.
Armed Forces Minister Al Carns may be coming up fast on the outside but like Sir Keir, before he became PM, he has limited political experience.
So Sir Olly’s smoking gun, if he has one, could well further wound the embattled Prime Minister but it may not deliver a fatal political blow.
Labour MPs are very unlikely to move against the PM just weeks before the May local elections when the party may lose as many as 2,000 councillors, be ousted from power in Wales, and get a drubbing in Scotland.
But in the aftermath of the polls, Sir Keir now looks less safe and there are few signs that his fortunes will improve.
