Propagation of the radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy leads to twinkling of these sources, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. intensity modulation of a few per cent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the Milky Way's interstellar plasma. So far, only three sources were known that show more extreme variations, with modulations at the level of tens of per cent on timescales less than an hour. This requires propagation through nearby (d10^2^cm^3^) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (~50%) and rapid variations on a timescale of just 6.5 minutes in the decimetre band (1.4GHz). The spatial scintillation pattern is highly anisotropic with a semi-minor axis of about 20,000 km. Canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sight-line to J1402+5347 however passes unusually close to the B3 star Alkaid (eta UMa) at a distance of 32pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor-axis of the scintels must be smaller than 10 microarcseconds yielding an apparent brightness temperature for an isotropic source of >10^14^K.