The story opens with Ginny Weasley awaking to mysterious find herself inside Mercy Hospital, a Muggle hospital in London. She soon remembers how she came to be there, although the description is too vague for the reader to draw much from it. Hearing she will have to get stitches, she convinces a friendly Muggle doctor named Dr. Gillian Hicks to let her call her parents.
In reality, she calls Harry at the Dursleys and explains her situation to him. After getting off the phone, Harry quickly sends an owl to the Weasleys to tell them where Ginny is. Meanwhile, back at the hospital, Ginny meets a nurse named Catherine, who is convinced that Ginny is really from another world of sorts.
However, that night the Dursleys entertain a "blue-blooded woman" named Diosa Puttidi and tell her that Harry is a mute and imply that he is severely mentally retardated. They use Harry's supposed condition to garner sympathy and money from Puttidi, but when the Weasleys arrive to pick up Harry, he drops the act and leaves with them aboard the Knight Bus.
Meanwhile, Ginny is interrogated by the extremely unpleasant Dr. Taylor, who appears to insane. Gillian Hicks, however, discovers his behavior, but, before she can do anything, Taylor kidnaps her and Ginny at gunpoint. After hearing Ginny's phone conversation with Harry, he has become convinced that Ginny has underworld connections and that he can use her to find out who she is working with.
The Weasleys and Harry, meanwhile, arrive at Mercy Hospital, where they meet Catherine. It is revealed that Catherine is actually a witch named Catherine Miller, who has been missing for many years. Arthur Weasley speculates that her memory was erased and that she was found by Muggles.
They then take the Knight Bus to rescue Ginny and, after finding her, destroy Taylor's car. An enraged Taylor threatens to kill Ginny if they do anything, but Catherine uses a Disarming Spell to knock his gun out of his hands and Arthur Weasley magically melts it. Taylor then runs out into the street where he is hit by a car and killed.
It is explained that the Muggle authorities were alerted to the accident and the story ends on the Knight Bus with Arthur Weasley checking that his daughter is all right.
The concept is taken from the film Star Trek IV: The Voyage Home where Chekov is taken to a hospital after being injured in 20th century San Francisco and subsequently rescued by the rest of the crew. This is reflected by the names used in the story. For example, "Mercy Hospital" is also the name of the hospital from The Voyage Home and the names Dr. Gillian Hicks, Catherine and Dr. Taylor are derived from the character Dr. Gillian Taylor and the actress who portrayed her, Catherine Hicks.
The story was followed by A Mission Like No Other, which was intended to explain how Ginny ended up in Mercy Hospital in the first place, but it was never finished.