The Brigadier asks the Doctor to investigate the disappearance of several scientists, only for him to find they have been abducted back in time.
- The story opens in medieval England, as a condottieri noble named Irongron and his men feast at their castle. Outside, they see a tremendous flash of light, and discover a spherical meteorite smoking in a crater. Inside is a bizarre man in armor, who introduces himself as Linx, of the Sontaran Empire. He claims the Earth for Sontar, and informs Irongron that his ship has been badly damaged, offering weaponry in exchange for Irongron's help in repairing the ship. Naturally, Irongron, although intrigued by the exotic arms that Linx offers, has no one who can repair such advanced technology, forcing the stranded Linx to try anoter tack. The action soon switches to a top-secret UNIT scientific base, where many of the leading scientific minds in Britain are being sequestered to prevent their disappearance. Among those placing himself in seclusion is the Doctor, attending both by the request of the Brigadier, and out of his own curiosity. He is lodged in a room with two other people: a mousy physicist named Reubish, and a young woman claiming to be the famed virologist Lavinia Smith, In short order, the Doctor ferrets out the truth -- the woman is not Lavinia Smith, but her niece Sarah Jane, an investigative journalist who used her aunt's credentials to sneak into the compound in the hopes that the vanishing scientists would make for a good story. After agreeing to keep her identity a secret, the Doctor gets to work trying to solve the mystery. He deduces that whatever is taking the scientists is using some sort of matter transporter device, and lays a trap by using a special Geiger Counter that detects the particles of the transferral device. From this, and a survey of Reubish's chambers after his own kidnapping, the Doctor uses the TARDIS to follow the particles to their source. Unbeknownst to him, Sarah Jane has already snuck aboard. Arriving in the Medieval period, Sarah Jane leaves the TARDIS, with the Doctor following close behind. After encountering a castle, Sarah Jane is abducted by one of Irongron's guards. Meanwhile, the Doctor follows the particle emissions to their source... Linx. As the episode closes, Linx removes his helmet, revealing a toad-like, misshapen face, like nothing from Earth...