  High-resolution spectroscopy has proven to be a powerful avenue for
atmospheric remote sensing of exoplanets. Recently, ESO commissioned the
CRIRES+ high-resolution infrared spectrograph at VLT. CRIRES+ is a
cross-dispersed spectrograph with high throughput and wide wavelength coverage
across the near-infrared (0.95-5.3 $\mu$m), designed to be particularly suited
for atmospheric characterisation of exoplanets. In this work, we report early
insights into the performance of CRIRES+ for exoplanet spectroscopy and conduct
a detailed assessment of the data reduction procedure. Because of the novelty
of the instrument, we perform two independent data reduction strategies, using
the official CR2RES pipeline and our new custom-built ExoRES pipeline. Using
science verification observations we find that the spectral resolving power of
CRIRES+ can reach $R \gtrsim 100,000$ for optimal observing conditions.
Similarly, we find the signal-to-noise ratio (S/N) to be consistent with
expected and empirical estimates for the observations considered. As a case
study, we perform the first application of CRIRES+ to the atmospheric
characterisation of an exoplanet - the ultra-hot Jupiter MASCARA-1 b. We detect
CO and H$_2$O in the atmosphere of MASCARA-1 b at a S/N of 12.9 and 5.3,
respectively, and a temperature inversion revealed through the CO and H$_2$O
emission lines, the first for an exoplanet. We find a combined S/N of 13.8 for
CO and H$_2$O together, with a preference for lower H$_2$O abundance compared
to CO. Our findings demonstrate the scientific potential of CRIRES+ and
highlight the excellent opportunity for high-resolution atmospheric
spectroscopy of diverse exoplanets.
