Sarah Jane, Maria, Luke, and Clyde investigate strange lights sighted near the village of Goblin's Copse, where a radio telescope observatory is located.
They discover Sontaran Commander Kaagh, the sole survivor of an invasion force involving the infiltration of Earth's cars that was otherwise destroyed,[N 1] and his space pod.
After receiving a phone call from his daughter Maria, Alan, secretly followed by his ex-wife Chrissie, asks for help from Mr Smith in Sarah Jane's attic.
After overhearing the Sontarans' weakness from Mr Smith, Chrissie heads to the observatory, and knocks Kaagh unconscious by striking the high heel of her shoe into the back of his neck.
[12] When Sarah Jane asks Mr Smith if he has acquired a sense of humor since his reboot, he replies, "I will run a diagnostics check immediately" whilst playing the sound effect associated with the Book from the television adaptation of The Hitchhiker's Guide to the Galaxy.
[16] The Jacksons were written out as regulars in The Sarah Jane Adventures because Yasmin Paige left the series in order to focus on her GCSEs.
"[23] He brands Clyde and Luke as "dependable as ever"[24] and the Jacksons as "endearing", the latter compensating "for [Part Two's] action failings" with Maria and Alan's departure being "well handled" and "touching" and Chrissie being "a revelation...[as] her seemingly vacuous nature has been replaced by an air of mystery.
"[23] Although Rawson-Jones notes "the subplot featuring the Sontaran-controlled Professor stalking his own daughter Lucy is rather disturbing for a CBBC show that airs at 4.35pm",[24] he describes the adventure as "[f]ittingly...death-free and surprisingly heartwarming.
Whilst praising Knight and Langer for being "much more fluid in their movements", Holm does not share Rawson-Jones's appreciation for the emotional scenes in "Part One" regarding Maria's departure and claims they lack "the resonance you might expect.
[27] Holm likens Sarah Jane's gang to Mystery, Inc. from Scooby-Doo and sees Kaagh as "a character somewhere between the sadistic Styre from The Sontaran Experiment and General Staal in "The Sonataran Stratagem".