Doctor Who S01E09 - PsychoFilms

http://www.imdb.com/title/tt0562996/

Rose and the Doctor find themselves in 1941 London when the TARDIS receives an emergency signal from another time-traveling vehicle. While the Doctor tries to determine where the object may have landed, Rose goes off in search of a child she can hear calling for help. When she gets herself into a precarious situation, she's rescued by the rather handsome RAF Captain Jack Harkness, who owns a time machine of his own. The Doctor meanwhile meets Nancy who has an interesting way of arranging meals for homeless children. They are also being chased by the child Rose heard calling. The Doctor determines that human DNA is being rewritten but it's not obvious why or by who.