An alien baby mysteriously appears on Sarah Jane's doorstep. Two different aliens come to Earth searching for her.
After talking to Luke on webcam, Sarah Jane is suffering from empty nest syndrome. All that changes when a baby girl mysteriously appears on her doorstep. The baby has the ability to cause power surges, destroying any nearby electric devices. Sarah Jane names the baby "Sky" because she "dropped out of the sky."
Two people come looking for the infant. The first is a metal man who lands in a junkyard, frightening a homeless man who lives there. The other is a woman named Miss Myers, who claims to be the child's mother and is leading "Fleshkind" in a war against the "Metalkind." She takes over a power plant and uses it to find the baby.
Sarah Jane and Rani leave to investigate the junkyard while Clyde stays at 13 Bannerman Road, looking after Sky. The Metalkind finds Sky and attacks Clyde, who is rescued by Miss Myers. Miss Myers takes him and Sky to the nuclear power plant. Sarah Jane and Rani return to 13 Bannerman Road to find Clyde missing. With Mr. Smith's help, they track him to the power plant.
Miss Myers demands that Clyde give her Sky so that the child can fulfill its "destiny." Not trusting her, Clyde runs and eludes her until Sarah Jane and Rani show up. However, the Metalkind also appears. Miss Myers reveals that the child is a powerful weapon and the Metalkind's presence has "primed" her. Sky then transforms from a baby into a teenage girl.
Miss Myers reveals that Sky is a weapon designed to destroy the Metalkind. Refusing to allow Sky to be used for genocide, Sarah Jane, Clyde, and Rani escape with Sky and bring her back to 13 Bannerman Road. Mr. Smith scans her and finds that her powers are meant to respond to the presence of Metalkind. The more Metalkind near her, the more powerful she becomes. Only Miss Myers has the ability to deactivate her powers. Sarah Jane and the others return to the power plant, hoping to convince Miss Myers to deactivate Sky's powers and find a peaceful solution to the war.
According to Mr. Smith, Sky's people come from a binary planetary system in which the Fleshkind and the Metalkind have been at war for centuries. Telling the kids to stay outside, Sarah Jane enters the power plant alone to talk to Miss Myers. When Sarah Jane confronts Miss Myers, she is told that the Metalkind have savagely attacked the Fleshkind. However, the Metalkind, who had been taken prisoner by Miss Myers, claims that the Fleshkind had taken Metalkind children as slaves. Miss Myers reveals that she will use the Metalkind as a homing beacon, creating a portal to bring the rest of his kind to Earth. This will show the humans the brutality of the Metalkind and cause Sky to detonate, killing her and the Metalkind.
Disobeying Sarah Jane, Sky follows her into the power plant, forcing Clyde and Rani to go with her. When Sky senses the Metalkind coming, she loses Rani and Clyde, intending to kill herself and save the world. Clyde and Rani manage to deactivate the nuclear reactor, cutting off Sky's power. This also shuts off the portal, stopping the Metalkind invasion. The Metalkind prisoner grabs Miss Myers and teleports away.
Sarah Jane brings Sky home with her, deciding to adopt her. Returning to the attic, they find the Shopkeeper and the Captain. The Shopkeeper reveals that he is the one who left baby Sky on Sarah Jane's doorstep but refuses to answer any of Sarah Jane's questions about him.
- Blatant Lies: "Would you believe it, they brought me a baby called Sky when it should have been a teenager, with the same name."
- Comically Missing the Point: "I think Sky just grew up" "With no birthdays? That's just rubbish"
- Cousin Oliver: Cute kid added during what ended up being the final episodes of the series, ostensibly as a successor to a longtime regular? Sky had potential to be the SJA version of this trope but, ended up averting it instead.
- Doorstop Baby: Sky
- Easy Amnesia: Miss Myers' henchman wakes up with no recollection of his actions under the influence.
- Evil Is Hammy:"The end is nigh!"
- Flesh Versus Steel: The Fleshkind versus the Metalkind almost literally names the trope.
- Go Look at the Distraction: Clyde looks over the shoulder of the metal man as if someone was behind him, the metal man turns to look giving Clyde the time to escape.
- Incredibly Lame Pun Several, but not all from Clyde this time
- "Take a power nap boys"
- "What do you call a man in paper trousers: Russel."
- Invisibility: The metal form is able to turn himself perfectly invisible, but it does not prevent him leaving footprints.
- Lampshade Hanging: "I was lucky, it was only the ten thousand and fifty-second camera I checked"
- Lightbulb Joke: Gita to her long suffering husband.
- Magic Pants: When Sky grows from a baby to a teenager, her clothes grow to match.
- Shout-Out: Two in the same conversation: "So you saw an iron mannote , like a robot.", and "Did I say he was a robot? No, I know what a robot looks like, I've seen Star Wars."
- "Get in if you want the child to live". Miss Myers also has some similarities in personality and plot role to Weaver from Terminator: The Sarah Connor Chronicles, and her portals look exactly like Terminator time-travel bubbles. Except without the nakedness. This is a kids' show after all.
- The Russell joke, while not original, is a probable reference to the show's creator, Russell T Davies.
- Sure, Let's Go with That: Sarah Jane accepts Gita's explanation of fostering for the sudden appearance of the baby.
- Take Me to Your Leader: Sarah Jane lets that drop on Miss Myers' henchman.
- Tyke Bomb: Sky