With Theresa May off in Bahrain painting her Turner prize-losing Brexit red, white and blue, the short straw of standing in for her at prime minister’s questions fell to the leader of the house. It was David Lidington’s bad luck to have found himself up against a forensic Emily Thornberry rather than the haphazard Jeremy Corbyn; it was his misjudgment to come to the house almost totally unprepared.

The shadow foreign secretary doesn’t normally cover herself in glory at the dispatch box, often managing to antagonise as many in her own party as she does on the government benches. But for her promotion to PMQs, she had come unusually well-primed. Her plan was nothing less than Lidington’s assassination.

Could the leader of the house give a simple answer to a simple question? Was Britain planning to stay in the customs union? Yes or no? Lidington appeared startled. Whatever instructions the prime minister had left him on her Post-it note, it hadn’t included this. He waved his arms around theatrically, as if hoping to drag up an answer with a little method thinking. Nothing. Nada. Customs union was well above his pay grade, but he was sure the government had a great plan.

Thornberry pressed him a little harder. Did he remember saying back in February that leaving the customs union would be a total disaster? “Does the minister still agree with himself?” she inquired, twisting the stiletto. The look of panic that crossed Lidington’s face suggested he remembered it only too well.

“Um, er,” he mumbled. “Things have changed a lot since then.” Certainly they had for Lidington, who was now entering a possible career-ending parallel universe. Someone passed him a note, which he eagerly opened. “The customs union is not a binary issue. There’s at least four possible outcomes.” He had no idea whether this was true, no idea of what it meant, but it was all he had to offer. The next time Theresa went away, she could get someone else to do her dirty work; up in the gallery, the prime minister’s advisers were having much the same thought.

Customs union was also on the agenda as part of the opposition day Brexit debate. Faced with the certainty of being defeated, the government had quickly tabled an amendment promising to publish some kind of plan before triggering article 50 in March next year, so the shadow Brexit secretary, Keir Starmer, tried to pin down the parameters of what might be in the plan. Could the government offer any clue about what sort of Brexit it had in mind?

“No plan survives engagement with the enemy,” Crispin Blunt snapped. Branding the EU as the enemy before negotiations have even started may not be the best way of securing the best possible deal.

David Davis, the Brexit secretary, tried to be a little more accommodating. He had been falling over himself in the past few months to let parliament know what the government plans. It was just that no one had been listening to him. His plans were to get the best possible deal for the country once someone had got round to working out what the best possible deal was. Whatever the government managed to come up with would de facto turn out to be the best possible deal.

When Conservative Dominic Grieve pointed out that if the supreme court upheld the divisional court’s ruling on parliamentary involvement in article 50, then the government would be obliged to introduce primary legislation, Davis grandly announced: “We will obey the rule of law.” It says something for post-truth politics in 2016 that Davis managed to make that sound like a concession.

What Davis didn’t say was what the plan the government would present to the Commons would look like. Would it be a few gentle hints written on the back of an envelope? He couldn’t say for certain. Though probably it would be nothing that detailed.

Thereafter the debate rather fell apart. The Eurosceptics, once the strongest defenders of parliamentary sovereignty, were now adamant that Brexit was far too important to be left to MPs and insisted that anyone who said otherwise was just trying to thwart the will of the people. As if they had a hotline to what that will was. The remainers were equally certain that allowing the government to stumble into any old Brexit within an arbitrary timeframe was asking for trouble.

“Would anyone in this house sign up to an agreement without knowing the details?” Starmer asked. “If so, please put your hand in the air.”

Jacob Rees-Mogg put his hand up. There’s always one.