Doctor Who, "The Waters of Mars"
David Tennant has had a fabulous run as The Doctor, and now the series is winding towards his closing time. Although "The Waters of Mars" doesn't directly lead into the last two Tennant episodes (aside from a tiny bit at the end) it represents a change in tone for the series, from "sprightly and fun" to "downright funereal."
The Doctor is travelling alone in "The Waters of Mars," and that never ends well for him. There's a reason why The Doctor chooses a companion. In this special, we get to see full well what happens when he goes banging about without an assistant to balance out the crazy and act as a moral compass. Frankly, he goes completely off the rails.