A hybrid drive system with first drive 11—especially a combustion engine—and second drive 12—especially an electric machine—for driving a transmission output shaft 20 of a transmission 10 for a vehicle is provided. The transmission 10 can include a first shiftable partial transmission 15 with a first main shaft 17, which is connectable by a first group of shiftable gear pairs 41/51, 45/52, 43/53, 44/54 to an intermediate shaft 28, and the first drive 11 is connectable to the first main shaft 17 of the first shiftable partial transmission 15. In addition, a second shiftable partial transmission 16 with a second main shaft 18, which is connectable by a second group of shiftable gear pairs 45/51, 46/52, 47/53, 48/54 to the intermediate shaft 28, can be included. At least one shiftable gear pair of the first shiftable partial transmission 15 is formed by two shiftable idlers 41, 51, a first of the two shiftable idlers 41 is connectable via a shifting element A to the first main shaft 17 and a second of the two shiftable idlers 51 is connectable via a shifting element C to the intermediate shaft 28.