Doctor: David Tennant (Tenth Doctor)
Companion: Freema Agyeman (Martha Jones)
Anne Reid – Florence Finnegan
Roy Marsden – Mr Stoker
Adjoa Andoh – Francine Jones
Gugu Mbatha-Raw – Tish Jones
Reggie Yates – Leo Jones
Trevor Laird – Clive Jones
Kimmi Richards – Annalise
Ben Righton – Oliver Morgenstern
Vineeta Rishi – Julia Swales
Paul Kasey – Judoon Leader
Nicholas Briggs – Judoon Voices
Directed by Charles Palmer
Written by Russell T Davies
Script editor Simon Winstone
Produced by Phil Collinson
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 3.1
Series Series 3
Length 45 minutes
Originally broadcast 31 March 2007
Medical student Martha Jones is on her way to work at a London hospital when she bumps into a strange man. He pauses in front of her and removes his tie, then walks away. When she arrives at work, she sees the same man as a patient under the name "John Smith". The man is unaware of seeing Martha earlier in the morning.
Suddenly the occupants of the hospital find that the entire building has been transported to the moon. Three spaceships land nearby, and the hospital is invaded by the Judoon. The Judoon are an intergalactic police force who are searching for a Plasmavore, a blood consuming alien with the ability to appear as the species whose blood it consumes. The Judoon begin scanning everyone in the hospital, cataloguing the humans whilst attempting to find the non-human criminal. John Smith, who turns out to be the Doctor, talks to Martha, revealing that he is an alien as well. The two go down to the lobby to find out what the Judoon are doing. An older lady named Florence Finnegan reveals that she is the Plasmavore and evades detection by sucking the blood of humans. She drains Mr. Stoker, the head of the hospital, and consequently registers as human when the Judoon scan her. The Doctor knows if he is scanned he will register as non-human, so he and Martha quietly avoid the Judoon, who will also likely execute all people within the hospital on grounds of harboring a fugitive if they discover a non-human in the building. As the oxygen level in the hospital drops, people begin to collapse. The Doctor and Martha are chased by Miss Finnegan's servants, leather clad bikers known as slabs. The Doctor uses his sonic screwdriver to ramp up an X-ray machine and destroy a slab, but the process also destroys his screwdriver.
The Doctor and Martha discover Stoker's body drained of blood, confirming the Doctor's suspicion of a Plasmavore in the hospital. To buy time from the advancing Judoon, the Doctor kisses Martha. This leaves a faint trace of non-human DNA on her, confusing the Judoon and forcing them to do a full scan on her. The Doctor finds Miss Finnegan in an MRI room, who is modifying the scanner to make it destroy all life on the moon and the half of the Earth currently facing it. The Doctor pretends to be a confused human and Miss Finnegan drinks his blood until he collapses. The Judoon arrive and confirm the non-human is deceased. Martha enters and grabs a Judoon scanner and exposes Finnegan as non-human. Confirming Finnegan is the fugitive they seek, the Judoon execute her for the murder of an alien princess. The Judoon declare their job done and leave, marching back to their ships. Martha uses CPR on the Doctor's two hearts and manages to revive him. He disables the modified MRI and the Judoon finally shift the hospital back to Earth seconds before everyone perishes from oxygen starvation.
That evening, Martha attends a birthday party for her brother that ends in a nasty family fight. While outside, she sees the Doctor and follows him to the TARDIS. He invites Martha to go on a trip with him, but she hesitates until the Doctor says he can travel in time as well. He proves it by travelling back to the previous morning, taking off his tie in front of Martha, then travelling back to show her the tie. Martha steps into the TARDIS, and the Doctor tells her that she is only going on one trip with him. The Doctor sets a course and the TARDIS sets off.