A new ternary polar intermetallic, Ca18Li5In25.07, was obtained from high-temperature reactions of the elements in welded Nb tubes. Its crystal structure, established by single-crystal X-ray diffraction, was found to crystallize in the orthorhombic space group Cmmm (No. 65). Unit cell parameters are a = 9.9151(6) Angstrom, b = 26.432(2) Angstrom, and c = 10.2116(6) Angstrom; Z = 2. The structure of Ca18Li5In25.07 features two distinct types of indium anionic layers. An "electron-deficient" layer is made up of Li-centered In-12 icosahedra that are interconnected by bridging planar In-4 units and In atoms. A second In-3(5-) layer is an electron-precise Zintl layer formed by fused four-, five-, and six-membered rings of three- and four-bonded indium atoms. The two distinct layers are alternately stacked and linked into a complex three-dimensional network. Vacancies are observed to occur only at the In-12 icosahedral and the bridging indium units within the "electron-deficient" layers. Magnetic property measurements indicate that Ca18Li5In25.07 exhibits temperature-independent paramagnetism consistent with metallic behavior. Band structure calculations were performed to elucidate the role of defects and vacancies in the electronic structure of the electron-deficient "metallic" Zintl phase.