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 impressive 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.
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.
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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.
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.
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.