After trying to scrub his teeth in the very mirrored bathroom of the TARDIS with his eyes open for once, the Doctor, who tends to sleep shirtless,
has discovered the Master's claiming scrawl all over his back.
Various friends and TARDIS denizens thus try to remove it.
Interestingly, NYC Jack, while playing scrub the Doctor, discusses that he's gotten the Junkyard a pet Ianto (Broken Ianto).
Also, the secret behind the Doctor's continued enjoyment of the Master's drugged muffins, knowing perfectly well that they're drugged, has come out.