Synopsis by Hal Erickson
In the third episode of the six-part story "Planet of the Spiders," three spider-like creatures from Metebelis 3 arrive on earth by way of an English monastery. There mission: To retrieve a powerful blue crystal, previously stolen from them by the Doctor (Jon Pertwee), which is highly coveted by the giant spider empress, the Great One. Unfortunately, the crystal is swiped from the Doctor's friend, Sarah (Elisabeth Sladen), by a crook named Lupton (John Dearth) -- who subsequently loses the valuable artifact to another crook named Tommy (John Kane). All of this has a profound effect on the Doctor, who knows that mankind will be destroyed unless the crystal is returned to its rightful owner. Written by Robert Sloman, "Planet of the Spiders, Episode 3" originally aired on May 18, 1974.