  Plasmons, collective oscillations of electron systems, can efficiently couple
light and electric current, and thus can be used to create sub-wavelength
photodetectors, radiation mixers, and on-chip spectrometers. Despite
considerable effort, it has proven challenging to implement plasmonic devices
operating at terahertz frequencies. The material capable to meet this challenge
is graphene as it supports long-lived electrically-tunable plasmons. Here we
demonstrate plasmon-assisted resonant detection of terahertz radiation by
antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot
cavities and rectifying elements. By varying the plasmon velocity using gate
voltage, we tune our detectors between multiple resonant modes and exploit this
functionality to measure plasmon wavelength and lifetime in bilayer graphene as
well as to probe collective modes in its moir\'e minibands. Our devices offer a
convenient tool for further plasmonic research that is often exceedingly
difficult under non-ambient conditions (e.g. cryogenic temperatures and strong
magnetic fields) and promise a viable route for various photonic applications.
