Participants with relapsed/refractory multiple myeloma can participate if all eligibility criteria are met. Tests required to determine eligibility include disease assessments, a physical exam, ECG and echocardiogram of the heart, brain MRI, and blood draws. Eligible participants have white blood cells collected by apheresis. These cells are genetically modified to make the experimental treatment KITE-585. Participants receive chemotherapy prior to the KITE-585 infusion. After the KITE-585 infusion, participants will be followed for side effects and effect of KITE-585 on their myeloma. Study procedures may be performed while hospitalized and/or in the outpatient setting.