Synopsis: The Doctor and Rose appear to have materialized underwater. As they investigate their surroundings, they discover a giant monster, which they destroy with the help of some mysterious organisms.
The story shifts focus to hone in on a blue-skinned woman—Queen Svelna—on an operating table. Her doctor claims that she's been miraculously healed. When they pull a sample of her blood, they discover that a mysterious blue box and two aliens have been in her bloodstream. The "mysterious organisms" from earlier in the story are thus revealed to have been the Queen's white blood cells, and the "giant monster" was some sort of virus.
Later, when the Queen has recovered, she places the blood sample under a powerful microscope, whereupon she meets the Doctor and Rose for the first time. The Doctor reminds the Queen that she's due to hammer out an important peace treaty, now that she's cured. She seems to acquiesce, then offers the Doctor a chance to be her royal consort. He says he's "not the marrying kind", then pops off in the TARDIS for another adventure, perhaps this time amongst people who are a little less gigantic than the Svelnoids.