Tesco has been Lowe London's ticking time-bomb for several months.
Now, Sir Frank Lowe has pushed the button and the repercussions will be profound.
Not just on the agency itself, which can't disguise the severity of losing its flagship £50 million Tesco account to Sir Frank's start-up.
There's also the secondary damage that will be inflicted on DDB London, should its "dream team" of Paul Hammersley, the chief executive, and David Hackworthy, the chief strategic officer, decide to join the new venture.
As in the aftermath of any explosion, confusion is rife. While Paul Weinberger, the longtime guardian of Tesco creativity, has quit the Lowe agency chairmanship to link up again with his former mentor, many questions remain unanswered.
Will Mark Cadman, Lowe's one-time senior suit on the Tesco business, give up the JWT managing directorship - and the prospect of promotion - to pledge his future to Sir Frank? Toby Hoare, JWT's executive chairman, is understood to want to keep Cadman but is unwilling to offer advancement under duress.
And what of Sir Martin Sorrell? The WPP chief has long coveted Tesco.
He's said to be interested in taking a stake in the new agency and handling its media buying (although nothing has been agreed).
NEW AGENCY'S CLIENTS
What other major business does the start-up expect to announce before the end of the year? Heineken is a possibility. Sir Frank is friendly with Freddie Heineken, the former Heineken boss, and his daughter Charlene de Carvalho, who has inherited a controlling stake in the brewer.
Insiders say the new agency will have to fight for that account, however. Lowe's Olympus account, meanwhile, looks to be an easier target. Sir Frank and Weinberger have maintained their relationship with Graham Chapman, the company's European marketing director.
WHAT IT MEANS FOR LOWE
For Lowe London, Tesco's defection, while not unexpected, could not have come at a worse time for the new management team, which had been working to stabilise the troubled agency. Tesco's £5 million income is said to account for more than 15 per cent of the agency's UK revenue and about 2 per cent of its global revenue.
The Nokia, Twinings, Ovaltine and Innocent Drinks wins, as well as entry on to the COI roster, have all been negated by the Tesco loss.
Redundancies will be inevitable, although they could be drastically reduced if, as expected, the agency's 20-strong Tesco team is given the chance to move with the business.
Some within Lowe London are angry that the network's Interpublic parent declined to mend fences with Sir Frank after his acrimonious exit two years ago by inviting him back in a non-executive role.
Now, with Tesco gone, it is expected that Tony Wright, the Lowe network's chief executive, will reconfigure the London agency.
Those close to Sir Frank say his new venture is not so much an act of revenge on IPG, more a symbol of his determination to be associated with an agency truer to his values than the one that bears his name. Indeed, Lowe London senior managers concede that the departure of Tesco and Weinberger marks the final break with the agency's past.
For the moment, though, mystery shrouds much of the new venture. One detail that has emerged is that the start-up, provisionally titled Lowe Weinberger & Partners, will not carry the Lowe name.
Nor is it expected that its founder, now 64, will take a day-to-day role or a much higher stake than his partners.
What isn't clear is what, if anything, the start-up will do about forging international links. Tesco has ambition beyond the UK and, perhaps, WPP could help it here. "It's not something that's being considered at the moment," a source close to Sir Frank said.
ADLAND'S VIEW OF ONE OF THE INDUSTRY'S MOST COLOURFUL VETERANS
Tim Bell former business partner of Sir Frank: "Frank is one of a handful of people who can move very large accounts. He's associated with some of the most outstanding ads this country has ever seen. He thinks there's life in the old dog yet, and he's probably right."
Alan Cluer celebrity Mr Fix-It: "Frank's troops will follow him anywhere, even if it's only out of curiosity."
David Jones former chief executive, Lowe Howard-Spink: "Frank has always been associated with the kind of witty and well-produced advertising that is all too rare these days. He's extraordinarily good at it and thinks there's a future for it. I hope for all our sakes that he is right. His heart has always been in the business and he always wants to see whether he can do it again."
Tim Lindsay former Lowe Worldwide president: "Frank has got a piece of wiring in his brain that enables him to assimilate the common touch. He knows how to hit the nation's collective nerve."
Matthew Bull chief creative officer, Lowe Worldwide: "Frank Lowe has always been like a business father to me. He's one of the most inspirational men you could ever be around and he has chosen fantastic partners for his new venture. I owe him an enormous amount."