This one is semi-canonical within the greater series. You see, back in the day when the new series was really new, then-producer Russell T. Davies commented on this story and said that as far as he was concerned, it was the opening shot of the Time War. It features the Daleks up to their usual levels of evil, and includes Lalla Ward returning to the series for the first time since the Fourth Doctor days playing President Romana. The real treat, here, though, is Maggie Stables playing the Sixth Doctors newest, audio-only companion, Evelyn Smythe. Smythe, a history professor in her mid-50s, is instrumental in helping the Doctor transition from the jerkass he was in the TV series to the milder, moderate character he comes in the later audios. Shes also just a wonderful character in her own right, and makes every story shes in something worth listening to.