There were goals, celebrations, cards, drama and suspense all rolled into an action-packed first half which the home side led 3-2 that eventually became the final scoreline with ATK heading Mumbai with their head held high for the second leg on Tuesday.
24-year-old midfielder from Mizoram Lalrindika Ralte (3rd) gave ATK a spectacular lead in front of a 12,500 capacity crowd but their joy was short-lived as they conceded successive goals in nine minutes.
Leo Costa (10th) and Gerson Vieira (19th) made full use of a Diego Forlan freekick as Mumbai in their maiden semifinal appearance bounced back in style.
But Hume derailed all their plans with a double strike in the 39th and 45+2nd minute as ATK's never say die attitude finally paid off.
There was more misery in store for Mumbai who were reduced to 10 players with none other than their Uruguayan superstar Forlan sent off in the 74th minute.
Having shown a yellow in the 51st minute, Forlan had to leave the field when referee Dilan Perera showed him a second with his malicious attack on Jewel Raja.
The 2010 World Cup hero tried to break from the left and had the ball taken from him by Raja and in frustration Forlan just kicked him to see his semifinal campaign end as Mumbai will miss him sorely in their home leg on December 13.
This was ATK's only second win at home this season but it came at the biggest stage to make them favourites to reach the final for a second time after 2014.
ATK thus made up for their unflattering outing at home as they had only one win and five draws and one loss at the league stage this season.
After Forlan was sent off Postiga scored in the 78th minute but only to be shown the offside flag.
Hume almost brought up his hattrick but his effort misses the far post in the 90+4th minute.
Kolkata started the game better and scored via Larindika Ralte in the third minute but within seven minutes they conceded when Leo Costa equalised with a good low drive beating the 37-year-old ATK goalkeeper Daniel Mello.
ATK straightaway pressed themselves into action when Ralte gifted an easy ball to Postiga and the Portuguese played to Hume who almost took the team ahead inside 30 seconds but the linesman raised the off-side flag.
But they did not have to wait long as Ralte became their first Indian player to score a goal this season in a move that resulted from a Doutie corner as a cleared came to Borja Fernandez.
The ATK captain's clever cross was flicked by a Ralte header that looped into top right corner as Mumbai goalkeeper Amrinder fumbled and the ball went in.
Soon it was Forlan who made the difference with his two freekicks resulting in Mumbai's two goals in nine minutes that helped them snatch a 2-1 lead within 19 minutes. The ATK surge was cut short much to the dismay of the home crowd.
ATK defence was caught napping from a Forlan free-kick that floated inside the box and India captain Sunil Chhetri header found Costa.
The Brazilian's launched a solid right footer that nestled into the far post with ATK's veteran goalkeeper looking clueless to make it 1-1.
Vieira got on the other end of a brilliant curling Forlan free kick and attacked from the close range as ATK's defence started to pale in comparison.
Things heated up a lot here and the referee flashed the yellow card more than once in the space of a few minutes -- to Lalrindika Ralte (23rd) and L'aihsanga Ralte (31st)
Soon, ATK seized the momentum with Mumbai struggling for possession that saw Hume bring the match on 2-all with Doutie setting him up brilliantly.
Helder Postiga made his way through the centre and a foul from Ralte saw Atletico win a penalty in the dying minutes of the first-half and Hume was bang on target deceiving Amrinder with ease from his 45+2nd minute spotkick.
Known for his defensive tactics, ATK coach Jose Molina earlier surprised with his line-up that had no Debjit Majumder who sat out for second successive match in a row as Mallo stood under the bar.
ATK also did not have first team regulars Abinash Ruidas, Javier Lara, Stephen Pearson and Pritam Kotal, in a weak defensive line-up that was evident when they conceded back to back goals in 10 minutes.