No companion, just the Doctor on Gallifrey. The president of the time lords has been assassinated and the Doctor has been framed. It turns out the Master is back, dying and decayed, and with the corrupted Chancellor, who actually killed the president, all so the Master could save himself from death and destroy Gallifrey. This is one of Tom Bakers best stories, if not the best, imo. The Master having run out of regenerations was a good way of explaining in universe why he's decayed and doesn't look like he used to. Great story.