Northampton had their five-game winning run brought to a crashing halt in Reading as London Irish prevailed 39-17 with a bonus-point on Saturday.
26 points in the 20 minutes either side of half-time helped the Exiles to a comfortable victory over the Aviva Premiership title hopefuls.
The Exiles were searching for their second win in six and looking to bounce from the previous week's defeat to Worcester, whole Northampton were flying following five wins on the bounce.
An inconspicuous start to the match suggested there'd be little between these two, but the form book was thrown out of the window in the latter three quarters as Irish ran away with the contest.
Jonathan Joseph got the ball rolling for Irish when he capitalised on a mistake from Vasily Artemiev to touch down, though the extras were missed.
Ten minutes later Steven Shingler put eight points between the sides with a penalty goal, before David Paice powered his way over the line for the hosts' second try. Shingler's conversion made it 15-0 with an Ian Humphreys drop-goal just before half-time put another three points between the two teams.
Northampton got off the mark six minutes into the second-half courtesy of a Stephen Myler drop-goal, but an unconverted Tomas O'Leary try minutes later re-asserted the Exiles' authority, before a Shingler penalty left Northampton with a mountain to climb.
Jamie Gibson secured the bonus-point for Irish on the hour with another good try, converted by Shingler, and, though the Saints rallied through tries from Ken Pisi and Phil Dowson in the final 15 minutes, it was too little too late for Saints, whose run was brought to a halt.
For London Irish:
Tries: Joseph, Paice, O'Leary, Gibson
Con: Shingler 2
Pen: Shingler 4
Tries: Pisi, Dowson
Con: Myler 2
London Irish: 15 Steven Shingler, 14 Topsy Ojo, 13 Jonathan Joseph, 12 Sailosi Tagicakibau, 11 Marland Yarde, 10 Ian Humphreys, 9 Tomás O'Leary, 8 Ofisa Treviranus, 7 Jamie Gibson, 6 Matt Garvey, 5 Bryn Evans (c), 4 George Skivington, 3 Halani Aulika, 2 David Paice, 1 Max Lahiff.
Replacements: 16 Shaun Malton, 17 Jerry Yanuyanutawa, 18 Alan Cotter, 19 Kieran Low, 20 Chris Hala'ufia, 21 Guy Armitage, 22 Shane Geraghty, 23 Jack Moates.
Northampton: 15 James Wilson, 14 Ken Pisi, 13 Dom Waldouck, 12 Luther Burrell, 11 Vasily Artemyev, 10 Stephen Myler, 9 Lee Dickson, 8 Phil Dowson (c), 7 Tom Wood, 6 Courtney Lawes, 5 Mark Sorenson, 4 Samu Manoa, 3 Brian Mujati, 2 Mike Haywood, 1 Soane Tonga'uiha.
Replacements: 16 Ross McMillan, 17 Alex Waller, 18 Paul Doran Jones, 19 Teimana Harrison, 20 Rhys Oakley, 21 Martin Roberts, 22 Ryan Lamb, 23 Tom May.
Referee: Mathieu Raynal (France)
Assistant referees: Kevin Stewart, Andrew Watson
Timekeeper: Eric Bullion
Assessor: Paul Bridgman