Complex sentence: On their way back to Earth aboard the Daedalus, a romance flourished between McKay and Keller, only to be cut short when Keller died soon afterward on Earth from exposure to the Hoffan drug.
Split into simpler sentences: On their way back to Earth aboard the Daedalus, a romance flourished between McKay and Keller. Things were going great until Keller died soon afterward on Earth from exposure to the Hoffan drug.