Plot summary
The night before her wedding to Rhys, Gwen is impregnated through being bitten by a male shapeshifting alien called a Nostrovite, which carries the fertilised egg before planting them in Gwen as a host body. Jack shoots the Nostrovite dead. On the day of the wedding, Gwen is almost at full term with the Nostrovite foetus. Toshiko is sent to the wedding venue to keep an eye on Gwen and to deliver a new wedding dress to accommodate her alien pregnancy. The male Nostrovite's female partner arrives at the venue intending to kill the foetus' host, and birthing it.
As Gwen and Rhys prepare to say their wedding vows, Jack runs up the aisle and demands that the wedding be halted. The half-eaten corpse of the wedding DJ Mervyn is discovered by the bridesmaid Megan, who alerts the entire wedding party to the murder. The Nostrovite reveals herself and is chased away by Jack, escaping through a window. Gwen tries shooting the Nostrovite, now disguised as Rhys' mother, but the maternal instinct has made her unstoppable.
Owen suggests using the singularity scalpel to destroy the alien foetus, after having done work on it since the Mayfly incident with Martha Jones. He sends Gwen to her room and teaches Rhys how to use the scalpel; Owen cannot use it himself due to injuries sustained in his undead state. When the shapeshifter arrives in the room disguised as Jack, Owen shoots her whilst Gwen exits with Rhys. Rhys and Gwen find refuge in a barn. As the Nostrovite launches an assault on the door of the barn, Rhys succeeds in destroying the foetus.
The Nostrovite enters, again impersonating Rhys' mother. She attacks Rhys, who attempts to protect himself with a chainsaw, which malfunctions. Jack arrives and kills the alien with a more powerful gun. Rhys and Gwen return to the wedding venue and are married. At the wedding reception, the guests all suddenly fall asleep. Jack reveals that he has drugged them all with amnesia pills mixed with sedative so that they will not remember the events of the day. Jack offers Gwen and Rhys the amnesia pill but Gwen declines as she does not want any secrets in her marriage to Rhys.
Credits
Adapted from Wikipedia contributors, Something Borrowed (Torchwood). Narrative and paragraph formatting adjusted; illustrations, reference markers and non-story material omitted.