Utopia is the eleventh episode of Series 3 and the first episode of a Mattress trilogy followed by The Sound of Drums and Last of the Time Lords. It marks the first appearance of The Mattress in NuWho, and only good one so far.
Hack Jarkness (John Barrowman) jumps on the TARDIS in an attempt to rape it. This scares the shit out of the TARDIS and it flies the Tenth Doctor (David Tennington) and Marty McFly (Gordon Freeman) all the way to the planet of Malcassairo in the year 100 trillion. There they meet a professor (played by the extremely based Derek Jacobi) who is trying to send the last of the human race to a place called "Utopia".
But uh-oh, it turns out the professor is none other than the Mattress! Also spoilers. After killing Chan-why-u-mad-tho he regenerates into John Simm and fucks off in the TARDIS to fuck Britain's shit up.
Pretty bloody good, if not GOAT.
Shame about how the story turned out in parts two and three. But they're non-canon like all Tennant finales.