A young woman (Martha) walks down a busy street fielding calls from her family when a strange man walks up to her and takes off his tie. She’s puzzled but continues to the hospital where she works. Next we see her among a group of medical students doing the rounds talking to an old lady and then a Mr Smith who she is asked to assess. She thinks he’s the man who took his tie off at her earlier but he denies it and suggests that she check with the nurses, he’s been here since yesterday. She listens to his heart and gets a surprise, Mr Smith winks at her and she says nothing, suspecting that it’s some kind of a joke.
The Consultant (Mr Stoker) gets a static shock from the patient’s notes and others say that they’ve had the same thing happen today. He says it’s because a thunder storm is coming in, and it’s Mr Smith who answers his question about who demonstrated it, but when he continues to say what a day it was and how he got soaked and electrocuted as it he had been there himself the Stoker mutters to one of the students that a visit from psychiatric might be needed.
Next Martha is on the phone to her sister again and while it’s dry where she is t’s pouring with rain at the hospital, only her sister is just around the corner. Turning it she sees a black storm cloud over the hospital and nowhere else.
More than that, the rain is going up not down. There’s a massive flash of lightning, a thunder clap and the whole hospital shakes. When they get up it’s dark outside, but it’s not night, they are on the moon. The whole hospital is on the moon. People start to panic.
Martha is calm and decides to open a window reasoning that it’s hardly airtight so something else is stopping all the air being sucked out. Behind her Mr Smith, now dressed, is impressed and suggests that they go outside to check it out.
The Doctor finally introduces himself, causing some confusion to student doctor Martha. He quickly establishes that they are inside a forcefield and unfortunately that means that the hospital has a limited air supply. Just then three enormous space ships arrive.
Masses of armoured figures disembark and start to march towards the hospital. Jadoon the Doctor calls them. Meanwhile the old lady, Mrs Finnegan, goes into Stoker’s office, she wants something, his blood.
The Jadoon march into the hospital and people scatter in panic. They start scanning everyone and drawing an X on their hands once they have been categorised as human.
Up stairs on a balcony the Doctor explains to Martha that the Jadoon sort of galactic police who have brought the hospital to the moon as it’s neutral territory, they have no jurisdiction on Earth. If they’re looking for someone non-human that’s bad news for the Doctor. Martha thinks he’s kidding.
As they work around the hospital one patient panics and hits a Jadoon. He is immediately charged with assault, found guilty and executed on the spot.
The Doctor looks through the hospital’s records trying to find whoever it is that the Jadoon are looking for. Unfortunately while he’s looking the Jadoon wipe the system.
Martha goes to see if Mr Stoker can help but walks in on Finnegan drinking blood from his corpse. She turns and runs, with Finnegan’s henchman in pursuit, and bumps into the Doctor. They sprint together along corridors until they get to the X-Ray department where the Doctor quickly modifies one of the machines and turns it on one of the henchmen ‘killing’ it. He then does a very odd little dance to collect the radiation he’s absorbed into his left shoe, which he then throws into a waste bin. Martha watches all this and then declares him to be completely mad.
But not to be distracted she tells him about Mrs Finnegan. The Doctor wonders why she’d be taking a snack now but then works out that she was assimilating the blood so that she’d be able to fool the Jadoon scan and appear human.
In another corridor Finnegan’s plan has worked, she scans as human and gets the cross on her hand. The Doctor on the other hand scans as non-human and they’re off and running again. They go to Stoker’s office but Finnegan isn’t there, just the Consultant’s pale, drained body. But what is Finnegan’s plan, she can pass as human but if the Jadoon don’t find what they’re looking for they’ll just execute everyone in the hospital, she must have some plan to escape. Then he notices a sign saying MRI. A group of Jadoon burst into the other end of the corridor and, first apologising and telling her that it means nothing, the Doctor kisses Martha. Then he leaves her to buy him some time and runs off.
In the MRI department Finnegan is tinkering. The Doctor walks in and plays dumb and starts nattering on about space rhinos. Finnegan falls for his act and tells him that the machine will send out a pulse that will fry the brain-stems of every living thing in 250,000 miles (including the half of the Earth facing the Moon) except her. With everyone dead she’ll be able to take one of the Jadoon ships and make her escape. The Doctor tells her that the Jadoon are going to increase their scans up to level 2 and she decides that she will have to assimilate again.
While all this is going on Martha is scanned and comes up as human but with non-human traces. She follows the Jadoon into the MRI department where they find Finnegan, confident that she’ll pass as human, and the Doctor lying dead. Martha realises what’s happened and grabs one of the scanners. Finnegan is non-human. Before they execute her she sets the MRI to overload. The Jadoon are not interested, their case is closed and they leave.
Martha’s only hope is to resuscitate the Doctor which she does before the dropping oxygen levels cause her to collapse. He pulls the plug on the machine and then carries her back to the ward. Suddenly it starts to rain outside, the Jadoon return the hospital to Earth, just where they took it from.
Martha’s sister finds her and she sees the Doctor walking away.
That night the planned family party ends in the expected fight between Martha’s parents and his girlfriend. The Doctor watches from a nearby corner and Martha walks over to investigate. Down a back alley the TARDIS is parked. The Doctor invites her to go for a little trip but she says she doesn’t have the time. He tells her that’s no problem, he can travel in time, which she refuses to believe, so he proves it.
But it’s just one trip to say thank you.