  Circinus X-1 (Cir X-1) is a neutron star binary with an elliptical orbit of
16.6~days. The source is unique for its extreme youth, providing a key to
understanding early binary evolution. However, its X-ray variability is too
complex to reach a clear interpretation. We conducted the first high cadence
(every 4 hours on average) observations covering one entire orbit using the
NICER X-ray telescope. The X-ray flux behavior can be divided into stable, dip,
and flaring phases. The X-ray spectra in all phases can be described by a
common model consisting of a partially covered disk black body emission and the
line features from a highly-ionized photo-ionized plasma. The spectral change
over the orbit is attributable to rapid changes of the partial covering medium
in the line-of-sight and gradual changes of the disk black body emission.
Emission lines of the H-like and He-like Mg, Si, S, and Fe are detected, most
prominently in the dip phase. The Fe emission lines change to absorption in the
course of the transition from the dip phase to the flaring phase. The estimated
ionization degree indicates no significant changes, suggesting that the
photo-ionized plasma is stable over the orbit. We propose a simple model in
which the disk black body emission is partially blocked by such a local medium
in the line-of-sight that has spatial structures depending on the azimuth of
the accretion disk. Emission lines upon the continuum emission are from the
photo-ionized plasma located outside of the blocking material.
