There are times when no amount of planning can prepare you for the completely unexpected.

The Liberals returned Monday for the fall sitting of Parliament with a plan to highlight what they see as their success over the past three years in making life better for the middle class, to focus attention on their efforts to diversify Canada's trading partnerships, and to remind voters that they have more money in their pockets today than they did under the Conservatives.

It took a single member of the government's backbench mere minutes to unravel those plans.

Leona Alleslev crossed the floor to join Andrew Scheer's opposition Conservatives, denouncing the prime minister's handling of the economy and his failure, as she put it, to defend Canada's interests at a time of unprecedented instability.

"This is not a strong economy. And our country needs strong leadership."

A defection from the government benches to the opposition is unusual no matter when it happens.

Conservative Leader Andrew Scheer says he's 'extremely proud' to welcome Leona Alleslev into the fold. 0:17

But the timing of Alleslev's defection, and her explicit criticism of Trudeau's leadership in her message, gave Scheer two things he desperately needed this week: a reprieve from answering even more questions about Maxime Bernier's defection from the Conservatives, and a human Exhibit A in his role as lead prosecutor making the case that the Liberals are killing this country's reputation and economic foundation.

"If, like Leona, you supported and trusted Justin Trudeau and the Liberals in the last election and you are frustrated, or even angry, with the ineffective leadership, know this," Scheer said directly into the cameras as Alleslev stood by his side. "You are both welcome and needed in the Conservative Party of Canada."

It's not clear how much damage Alleslev's surprise defection will cause the Liberals, but it certainly made for an awkward start to the fall sitting.

The government had selected, as its first item of business, continuing debate on the bill to ratify the Pacific rim trade deal known as the Comprehensive and Progressive Trans-Pacific Partnership.

The choice was calculated. Conservatives support the deal. It also serves to showcase the government's efforts to diversify this country's trade ties with Asian and European nations as NAFTA negotiations drag on — and while U.S. President Donald Trump tries to ratchet up pressure on Canada to sign a deal on his terms by threatening to add tariffs on Canadian-made autos to the ones he's already imposed on steel and aluminum.

The government also had prepared for questions on the future of the Trans Mountain Pipeline, having planned an announcement for the near future on how it will formally respond to a court ruling that killed the project's permits by ruling that First Nations hadn't been adequately consulted.

These are the major challenges the Liberals face. It's where they wanted the focus to be on day one. It's what they planned for as the Commons resumed.

Instead, the PM found himself responding to Scheer's allegation that he had presided over a "summer of failure" and that he "just isn't up to the job" of guiding Canada through difficult times.

The return of 'just not ready'

That last bit was a core theme of the unsuccessful Conservative re-election campaign of three years back. It didn't work then, even though Stephen Harper had a long track record in office. Conservatives seem to think it will work now, under an untested Scheer — a bold move.

What the argument doesn't do, as Liberals pointed out Monday, is tell Canadians what Scheer might do differently.

How, for example, would Scheer have approached dealing with Donald Trump during these tense NAFTA negotiations? How would Conservatives have succeeded in getting the Trans Mountain expansion built after failing to get the project going while in power for nearly a decade? And where, oh where, is the Conservatives' alternative plan to reduce carbon emissions without putting a price on emissions?

Liberals insist they have a good news story to tell on the economy during their three years in office. And Trudeau repeated the lines again on Monday during question period: the lowest unemployment rate in nearly four decades, the creation of 500,000 full-time jobs and the fastest economic growth among G7 nations last year.

The limits of the Liberals' message

But none of this seems very forward-looking for a government that will be asking voters for another mandate a year from now. Some Liberals admit privately that none of it is the sort of stuff to excite the party's base in a critical period when party nominations are underway and when fundraising is becoming more and more important.

Some even worry that the government is inadvertently providing an opening — slim as it might be — for the New Democrats.

NDP Leader Jagmeet Singh told reporters on Monday that his party will continue to confront the government with the stories they heard from ordinary Canadians over the summer about their troubles finding affordable housing, or paying for medication.

"While Liberals keep making empty promises that they won't keep … New Democrats are standing up for what matters most to Canadians."

The Liberals expected to be attacked from both sides, of course. What they didn't anticipate was an attack from within. And that requires a new plan for the days ahead.