 How did Ninian, Raymond, and the other cousins go back in time?
When Martin was sixteen, Raymond took him aside for the talk Ninian had promised five years before. In that way, there would be no space-drive, and, hence, the Terrestrials would never get to the other planets and oppress the local aborigines. "I wormed the whole thing out of him and all of us—the other cousins and me—held a council of war, as it were, and we decided it was our moral duty to go back in time ourselves and protect you." "Well, five years is rather a long stretch for any girl to spend in exile," Raymond explained, "even though our life spans are a bit longer than yours.
 They bribed the assistant for the plans and blackmailed or tortured someone to build the time transmitter for them.
 They bribed the assistant for the plans and blackmailed or tortured someone to build the time transmitter for them.  Professor Farkas' assistant sent them back in time using the time transmitter after they gave him a bribe.  They bribed the assistant for the plans and hired a gadget enthusiast to build the time transmitter for them.  Professor Farkas sent them back in time with the time transmitter.