We numerically investigate structural and optical properties of a highly chiral liquid crystal when it is confined in a thin planar cell imposing strong homeotropic anchoring. Various stable exotic defect structures different from those of bulk cholesteric blue phases are found depending on temperature and cell thickness. We also study how a planar cell with these defect structures reflect and transmit incident light, paying particular attention to a structure with double-helix disclination lines and one similar to bulk BP II.