The generation of anti-Stokes emission through lanthanide-doped upconversion nanoparticles is of great importance for technological applications in energy harvesting, bioimaging and optical cryptography. However, the weak absorption and long radiative lifetimes of upconversion nanoparticles may significantly limit their use in imaging and labelling applications in which a fast spontaneous emission rate is essential. Here, we report the direct observation of upconversion superburst with directional, fast and ultrabright luminescence by coupling gap plasmon modes to nanoparticle emitters. Through precise control over the nanoparticle's local density of state, we achieve emission amplification by four to five orders of magnitude and a 166-fold rate increase in spontaneous emission. We also demonstrate that tailoring the mode of the plasmonic cavity permits active control over the colour output of upconversion emission. These findings may benefit the future development of rapid nonlinear image scanning nanoscopy and open up the possibility of constructing high-frequency, single-photon emitters driven by telecommunication wavelengths.