The Gruithuisen region in northern Oceanus Procellarum on the Moon contains three distinctive domes interpreted as nonmare volcanic features of Imbrian age. A 4 d extravehicular activity (EVA), four-astronaut sortie mission to explore these enigmatic features and the surrounding terrain provides the opportunity to address key outstanding lunar science questions. The landing site is on the mare south of Gruithuisen γ (36.22°N, 40.60°W). From this site, diverse geologic terrains and features are accessible, including highlands, dome material, mare basalts, multiple craters, small rilles, and a negative topographic feature of unknown origin. Preliminary mission planning is based on Clementine multispectral data, Lunar Prospector geochemical estimates, and high-resolution (0.5 m/pixel) stereo images from the Lunar Reconnaissance Orbiter Narrow Angle Camera. Science objectives for the mission include: (1) determining the nature of the domes, (2) identifying and measuring the distribution of any potassium, rare earth elements, and phosphorus (KREEP)- and thorium-rich materials, (3) collecting samples for age dating of key units to investigate the evolution of the region, and (4) deploying a passive seismic grid as part of a global lunar network. Satisfying the science objectives requires 7 h, ̃20 km round-trip EVAs, and signifi cant time driving on slopes up to ̃15°.