This is a preliminary version of a formal proposal by the 3M collaboration to construct a megaton, modular, multipurpose (3M) neutrino detector for a program of experiments in neutrino physics. The detector components will be located in chambers approximately 7000 ft below the Earths surface in the Homestake Mine at Lead, South Dakota, to carry out experiments on neutrino oscillations directed toward the principal experimental goal of the program, viz., the issue of CP-invariance violation in the lepton sector of elementary particles, an issue that has been the subject of study in the quark sector for several decades. The principal physics goal of this program also requires a moderately intense neutrino beam from an accelerator located a long distance from the detector array, such as the 2540 km distance of BNL from Homestake. The construction plan for that neutrino beam is at http://nwg.phy.bnl.gov/. Other experimental searches that do not require the accelerator-generated beam can be carried out with the 3M detector independently of and at the same time as the neutrino oscillation and CP-invariance violation measurements are in progress. They are searches for Proton Decay, UHE Neutrinos, and Supernovae Neutrinos.