Doungel scored in the 42nd, 46th and 68th minutes to hand NorthEast United their biggest win in this edition of the ISL. The 24-year-old player from Manipur also became the first Indian player to score a hat-trick this edition.
Anirudh Thapa pulled one back for the Chennaiyin in the 79th minute but buy then the match at the Indira Gandhi Athletic Stadium had tilted heavily in favour of the Northeastern side.
Despite today's win, their second victory at home and third overall so far, NorthEast United remained at ninth spot in the 10-team league table with 10 points from 10 matches.
NorthEast United had lost 0-3 against Chennaiyin FC in Chennai earlier in their first-leg clash but Doungel turned the tables on their rivals with a hat-trick, the first ever for NorthEast since inception in 2014.
More From This Section
The Northeast side tore the Chennaiyin's defence to shreds to avenge their first-leg defeat.
NorthEast United had scored just four goals in their nine matches prior to this clash against Chennaiyin and it looked like they would draw a blank again.
Initially, there was urgency but no direction to the home team's attack and most moves fizzled out before reaching the danger zone.
The home team, though, were rewarded for their efforts in the 42nd minute as Doungel made the first attempt on goal count.
A ball just outside the box from Reagan Singh saw Doungel chest it back to Cezario. The striker weaved his way past two markers and fired a shot which Chennaiyin goalkeeper Karanjit Singh did not collect cleanly and Doungel slotted it home from close range.
Nineteen seconds into the start of second session, NorthEast United and Doungel were celebrating again with their second goal. A through ball from Danilo Lopes saw Doungel get to the end of it and finish it past the onrushing goalkeeper.
Chennaiyin FC, unbeaten for five matches, were stunned. Anirudh Thapa pulled a goal back in the 79th minute with a brilliant volley.