at the end of "trial", it was revealed that peri had married king yrcanos and settled on the planet krontep. the graphic novel "the age of chaos", written by colin baker (who played the sixth doctor) revealed that frobisher and the sixth doctor visited peri's descendants several times, so frobisher must have rejoined the doctor after "trial" in an unseen story. after this story, frobisher apparently departs again, since he is never again seen travelling with the sixth doctor.