Kirk's brother Sam was a research biologist on Deneva. When vessels from Ingraham B arrived carrying neural parasites, many of the colonists were infected, including Sam. Although his wife managed to send a distress call to the Enterprise, Sam died before help could arrive.
An ion storm causes a transporter malfunction, and the landing team instead materialize aboard an unfamiliar Enterprise. Their alternate universe counterparts simultaneously materialise on the Enterprise in the Federation universe, and are thrown in the brig by Mr Spock who immediately recognizes the changed personalities of the barbarian landing party.