The Cobblers three-game winning streak was ground into the Derbyshire dust as Chesterfield scored three goals in seven first-half minutes.
Former Sixfields striker Marc Richards scored twice as the Spireites won 3-0, all three goals coming in a first half that was as poor a 45 minutes as the team has produced this season.
Given the team’s poor away record this season, to lose a game on the road was no real surprise but the nature of the defeat will be of major concern to manager Aidy Boothroyd, whose side went into the game in red-hot form but delivered a cold and lifeless performance.
They struggled to get to grips with Chesterfield’s lone-striker system that allowed extra runners from midfield to join in attacks with total freedom, and made a string of bad mistakes in defence having been left exposed.
The first meaningful piece of action came on the quarter-hour mark when Richards curled a free-kick goalwards after a Kelvin Langmead foul only to see Lee Nicholls get across to his left and hold the shot.
It was a sign of things to come and the keeper was picking the ball out of his net moments later when Danny Whitaker rolled a pass into Jay O’Shea, who twisted and turned in the box before lashing a shot past Nicholls.
One became two almost instantly when Clarke Carlisle failed to deal with a ball over the top, Richards advanced with it and slotted past Nicholls with consummate ease.
Chesterfield were at full tilt by that point and should have had a third but two blocks in the penalty area, the first from Kelvin Langmead and the second from Joe Widdowson denied them.
But they did not have to wait long to extend their advantage. Luke Guttridge played an uncharacteristically sloppy pass which was cut off by Tendayi Darikwa and sprayed to Richards, who outpaced Langmead down the outside and shot across Nicholls into the corner.
Northampton required an urgent response and forced a save from home keeper Tommy Lee when Adebayo Akinfenwa got a shot away from a Ben Tozer throw.
But their attacks only ever came in bits and pieces, there was none of the consistency in midfield that had been the cornerstone of the previous three matches, all of which ended in victories.
With five minutes of the first half Craig Clay drew a good save from Nicholls after yet more good work by the outstanding Richards, who revelled in his role as the hosts’ lone striker with three midfield players behind him joining in at various intervals.
Town started the second half in the manner of a team with its manager’s words ringing in their ears but either spurned responsibility when in shooting positions or played with nowhere near enough accuracy to trouble the home defence.
Chesterfield could have scored twice more before the Cobblers had their first chance of the second wide, Clive Platt skewing one wide with his right foot when he really should have used his left.
Chris Hackett, pushed into a central role, found space for a shot just inside the box that hit the outside of the post in what was Northampton’s best chance up to that point and Ben Harding drew a good save from Tommy Lee with a free-kick low to his left-hand post.
Any goal would have been a consolation, though, and was unlikely to affect a scoreline to which Whitaker should have added when he pulled a shot high over the bar when well placed to increase the home side’s advantage.
Neal Trotman hit a post with a header two minutes from time as Chesterfield looked to extend their advantage to a margin of victory that their superiority deserved.
As it was, they had to settle for a 3-0 win but this was a comfortable win for the home side, who were unfortunate not to win by more.
Chesterfield: Lee, Hird, Trotman, Cooper, Smith, Darikwa, Whitaker, Togwell, Clay, O’Shea, Richards
Subs: Randall (for Clay 76), Lester (for Richards 85), Wafula (for O’Shea 90)
Not used: O’Donnell, Boden, Broadhead, Whittaker
Cobblers: Nicholls, Tozer, Carlisle, Langmead, Widdowson, Hackett, Guttridge, Harding, Robinson, Platt, Akinfenwa
Subs: Hornby (for Akinfenwa 74), Demontagnac (for Harding 82)
Not used: Snedker, Wilson, Moult, Dias, Turnbull