A legendary Babylonian dragon Zook existed in another dimension which was, oddly, sealed in a bio-dome. It seems there was a weak point between Earth and Zook's world located in Boston, Massachusetts, which in modern times had a subway system built over it.
It seems every several years Zook needed to feed off the lifeforce of others to survive; when he needed sustenance Zook would transport any unwary person into his realm. In order to aid him in capturing victims, Zook employed the Appleby family, who Zook gave immortality.
Role in the episode
In the episode "Mystery of the Vanishing", while investigating the disappearance of a conductor named Joe Green who had vanished in the Boston subway, Martin, Diana and Java were transported to Zook's dimension. Here they met the Applebys, who had feigned kindness to the trio. Drugging Martin, Diana and Java, the Applebys had planned on feeding them to Zook, but first they had planned on feeding the captured Joe Green to him.
Following the Applebys, the trio were ambushed and captured. The Applebys then tied Martin, Diana, Java and Joe Green up and left them to be eaten by Zook. Fortunately Martin managed to destroy Zook by distracting the dragon until its allotted feeding time ran out. With its time up and without lifeforce, Zook exploded which aged the Applebys.