Claude Puel exacted revenge on his first return to Southampton as Leicester romped to a 4-1 Premier League win at St Mary's.
Shinji Okazaki's brace and further goals from Riyad Mahrez and Andy King eased Leicester past the struggling Saints, who could only muster a scrappy effort from Maya Yoshida.
Frenchman Puel was sacked in the summer despite last term leading Southampton to an eighth-place finish and the League Cup final.
The former Nice boss was out to disprove accusations he had favoured boring football in his Saints tenure, and revelled in his Leicester charges laying on plenty of entertainment.
The same shortcomings that littered Puel's Saints tenure were on display for Mauricio Pellegrino's men however, with the hosts finding no fluency in attack and defending pitifully.
New boss Pellegrino's Southampton stint now appears under real strain: Saints boast just one win and six points in their last eight Premier League games.
After 17 games under Puel last term, Southampton had amassed 24 points - under Pellegrino at the same stage, they have just 18.
Leicester's joyful travelling fans seized on the chance to mock their hosts, chirping up with "Aren't you glad you sacked Puel?".
Toothless in attack and disorganised at the back: Saints' performance mirrored that of many under Puel last term.
Mahrez could hardly believe the time and space afforded to him by Southampton's defence as he cantered through the middle and rifled Leicester's facile opener.
Okazaki had one strike palmed away by Forster, then another cleared off the line by Mario Lemina.
Charlie Austin at least forced Kasper Schmeichel into a save but after an ineffective corner Saints were on the back foot in seconds.
Schmeichel's fizzing ball sent Jamie Vardy through the middle and the England man found Mahrez down the left flank. Only Forster's sharp save denied Vardy a fine goal after he redirected Mahrez's low centre.
Leicester hardly had to work to double their lead, a deep corner catching Saints napping and Okazaki poking in at the second attempt.
And when the visitors swept home their third, the home faithful headed indoors in their droves some seven minutes from half-time, desperate for anything other than Saints' miserable performance.
Harry Maguire's expert chest-down and spin bamboozled Saints' defence such that no one cleared his low cross - leaving the unmarked King with the tap-in honours at the far post.
Pellegrino sacrificed holding midfielder Oriol Romeu for striker Manolo Gabbiadini at the break but little changed before the hour.
Yoshida's header found its way in to cut the deficit, but the unconvinced home fans could only chorus their amazement at having found the net.
Austin should have doubled the re-energised hosts' tally when sliding onto the overlapping Ryan Bertrand's low cross only to side-foot too close to Schmeichel.
Leicester quickly killed any delusions of a Saints recovery with another clinical strike. Mahrez pounced on a loose pass, feeding Vardy, whose low cross was easily swept home by Okazaki.