  Investigating Protostellar Accretion is a Cycle 1 JWST program using the
NIRSpec+MIRI integral field units to obtain 2.9--28 $\mu$m spectral cubes of
five young protostars with luminosities of 0.2-10,000 L$_{\odot}$ in their
primary accretion phase. This paper introduces the NIRSpec 2.9--5.3 $\mu$m data
of the inner 840-9000 au with spatial resolutions from 28-300 au. The spectra
show rising continuum emission; deep ice absorption; emission from H$_{2}$,
H~I, and [Fe~II]; and the CO fundamental series in emission and absorption.
Maps of the continuum emission show scattered light cavities for all five
protostars. In the cavities, collimated jets are detected in [Fe~II] for the
four $< 320$~L$_{\odot}$ protostars, two of which are additionally traced in
Br-$\alpha$. Knots of [Fe~II] emission are detected toward the most luminous
protostar, and knots of [FeII] emission with dynamical times of $< 30$~yrs are
found in the jets of the others. While only one jet is traced in H$_2$, knots
of H$_2$ and CO are detected in the jets of four protostars. H$_2$ is seen
extending through the cavities, showing that they are filled by warm molecular
gas. Bright H$_2$ emission is seen along the walls of a single cavity, while in
three cavities narrow shells of H$_2$ emission are found, one of which has an
[Fe~II] knot at its apex. These data show cavities containing collimated jets
traced in atomic/ionic gas surrounded by warm molecular gas in a wide-angle
wind and/or gas accelerated by bow shocks in the jets.
