Controlling, detecting and generating propagating plasmons by all-electrical means is at the heart of on-chip nano-optical processing(1-3). Graphene carries long-lived plasmons that are extremely confined and controllable by electrostatic fields(4-7) however, electrical detection of propagating plasmons in graphene has not yet been realized. Here, we present an all-graphene mid-infrared plasmon detector operating at room temperature, where a single graphene sheet serves simultaneously as the plasmonic medium and detector. Rather than achieving detection via added optoelectronic materials, as is typically done in other plasmonic systems(8-15), our device converts the natural decay product of the plasmon-electronic heat-directly into a voltage through the thermoelectric effect(16,17). We employ two local gates to fully tune the thermoelectric and plasmonic behaviour of the graphene. High-resolution real-space photocurrent maps are used to investigate the plasmon propagation and interference, decay, thermal diffusion, and thermoelectric generation.