Arcus provides high-resolution soft X-ray spectroscopy in the 12–50 Å bandpass with unprecedented sensitivity, including spectral resolution over 2500 and effective area over 200 cm2. The mission has three top science goals for Arcus, which are (1) to measure the effects of structure formation imprinted upon the hot baryons that are predicted to lie in extended halos around galaxies, (2) to trace the propagation of outflowing mass, energy, and momentum from the vicinity of the black hole to extragalactic scales as a measure of their feedback, and (3) to explore how stars form and evolve. Arcus will also detect and characterize diffuse gas and interstellar dust in the Galaxy and halo via absorption spectroscopy towards Galactic X-ray binaries and distant AGN. Arcus uses the same 12 m focal length grazing-incidence Silicon Pore X-ray Optics (SPOs) that ESA has developed for the Athena mission; the focal length is achieved on orbit via an extendable optical bench. The focused X-rays from these optics are diffracted by high-efficiency Critical-Angle Transmission (CAT) gratings developed by the same group that built the Chandra HETG gratings. The resulting dispersed photons are imaged with flight-proven CCD detectors and electronics. Combined with the high-heritage NGIS LEOStar-2 spacecraft and launched into 4:1 lunar resonant orbit, Arcus provides high sensitivity and high efficiency observing of a wide range of astrophysical sources.