  We present the results from an Australian Square Kilometre Array Pathfinder
search for radio variables on timescales of hours. We conducted an untargeted
search over a 30 deg$^2$ field, with multiple 10-hour observations separated by
days to months, at a central frequency of 945 MHz. We discovered six rapid
scintillators from 15-minute model-subtracted images with sensitivity of $\sim
200\,\mu$Jy/beam; two of them are extreme intra-hour variables with modulation
indices up to $\sim 40\%$ and timescales as short as tens of minutes. Five of
the variables are in a linear arrangement on the sky with angular width $\sim
1$ arcmin and length $\sim 2$ degrees, revealing the existence of a huge plasma
filament in front of them. We derived kinematic models of this plasma from the
annual modulation of the scintillation rate of our sources, and we estimated
its likely physical properties: a distance of $\sim 4$ pc and length of $\sim
0.1$ pc. The characteristics we observe for the scattering screen are
incompatible with published suggestions for the origin of intra-hour
variability leading us to propose a new picture in which the underlying
phenomenon is a cold tidal stream. This is the first time that multiple
scintillators have been detected behind the same plasma screen, giving direct
insight into the geometry of the scattering medium responsible for enhanced
scintillation.
