Gloucestershire lost 7-34 with the second new ball after tea on day one at Northampton to be bowled out far short of the commanding first-innings total they should have made.
242-1 was the score shortly before tea when Chris Dent, on 92, ran past a James Middlebrook delivery to bowled - his dismissal ended a record second wicket partnership and sparked a remarkable collapse.
Alex Gidman was incredibly fidgety at the wicket and his feet went nowhere as he was bowled for just one. The second new ball then claimed 7-34 as Gloucestershire's dominating afternoon descended into a modest total.
No one could stop the rot. After Porterfield and Dent only Chris Taylor (21 not out) and Jon Lewis (11) limped out of double figures.
There was more drama to come as Gemaal Hussain, who had Steven Peters well caught by Porterfield, and Jon Lewis, who produced an edge from nightwatchman Middleborough, made a double-breakthrough before the close.