An electrically operated aerosol-generating system is provided, including an aerosol-generating device, a removable aerosol-forming cartridge, and a removable heater provided separately from the cartridge. The cartridge includes at least one aerosol-forming substrate; the heater includes at least one electric heater element and first electrical contacts connected to the heater element; the device includes a main body defining a main cavity and at least one opening configured to receive the cartridge and the heater within the cavity, and an electric power supply and second electrical contacts connected thereto. When the cartridge and the heater are received within the cavity, the first and second electrical contacts contact each other and the heater is configured to heat the substrate, the cartridge and the heater are substantially flat, and the cavity, the cartridge, and the heater are arranged so that the cartridge and the heater are substantially parallel and adjacent to each other.