  We introduce the unique class of propagation-invariant surface plasmon
polaritons (SPPs) representing pulsed surface wave packets propagating along
unpatterned metal-dielectric interfaces and are localized in all dimensions -
with potentially subwavelength transverse spatial widths. The characteristic
features of such linear diffraction-free, dispersion-free `plasmonic bullets'
stem from tight spatio-temporal correlations incorporated into the SPP spectral
support domain, and we thus call them `space-time' SPPs. We show that the group
velocity of space-time SPP wave packets can be readily tuned to subluminal,
superluminal, and even negative values by tailoring the spatio-temporal field
structure independently of any material properties. We present an analytical
framework and numerical simulations for the propagation of space-time SPPs in
comparison with traditional pulsed SPPs whose spatial and temporal degrees of
freedom are separable, thereby verifying the propagation-invariance of the
former.
