Dave Porecki's last-gasp try prevented 14-man Northampton Saints from going top of the Premiership as London Irish secured a thrilling 20-16 win at Franklin's Gardens.
The visitors were 16-8 down heading into the final eight minutes but tries from Franco van der Merwe and Porecki got the job done for Irish after Api Ratuniyarawa had seen red.
Ollie Hassell-Collins' early try gave the underdogs the lead, but the Saints – who lost England centre Piers Francis to injury in the first half – led at the break after Cobus Reinach dotted down and James Grayson added eight points with the boot.
Ratuniyarawa's second-half dismissal for a high clearance at a ruck proved pivotal, even if another Grayson penalty did initially give Northampton further breathing space.
Having seen Reinbach, who confirmed this month he would leave Saints at the end of the season, make a stunning try-saving tackle, the hosts' resolve was broken when Van der Merwe pierced the line.
Irish continued to press and got their reward at the death, Porecki pouncing to earn their first win since November after an attempted clearance was charged down.