HPA is one of the many secretive projects created by the Office of Naval Intelligence. Utilizing candidates originally planned for the SPARTAN-III Project, they carry out assassinations and other covert ops under total secrecy and with no remorse. They are killers to the bone, with no limitations on how to conduct their business. However, when one of those missions intercepts an operation conducted by the UNSCDF-MC SPARTAN-141, sparks fly as the lead agent and the enigmatic SPARTAN-II clash.