This trial will study SEA-BCMA to find out whether it is an effective treatment for multiple myeloma (MM) and what side effects (unwanted effects) may occur.
The study will have two parts. In the first part, participants get SEA-BCMA by itself. This part of the study will find out how much SEA-BCMA should be given for treatment and how often. It will also find out how safe the treatment is and how well it works.
In the second part of the study, participants will be given both SEA-BCMA and dexamethasone. Dexamethasone is a drug that can be used to treat multiple myeloma. The part will study whether SEA-BCMA and dexamethasone are safe when used together.