Doc calls in his remaining aids, Renny, Johnny and Long Tom for their help. The three aids follow a lead that runs to a strange encounter with a white-haired old man with the spry agility of a teenager. The old man says his name is Dan Thunden and claims to be 131 years old! Thunden pummels the three aids and escapes into the night. Later, Johnny investigates and finds that a Captain Dan Thunden of the schooner Sea Nymph was lost at sea 91 years earlier!

Doc, Monk and Ham are surprised when Doc's cousin Patricia Savage shows up. Having lost her father in The Brand Of The Warewolf, Pat has decided to move to New York city and start a business.  Doc allows Pat to "tag along", but mostly because it's just too hard to get rid of her. Within 30 minutes, Pat is swept up in two fights and exclaims, "Talk about leading violent lives!" All four set out to intercept Kel Avery at the airport before Santini's men can get to her. Pat tricks Santini and his men into thinking she is Kel Avery and Kel is her valet. The girls are taken prisoner. Pat takes quite a bit of rough treatment at the hands of Santini and his thugs. She is shot at, kidnapped, tied-up, threatened at gun-point and hauled from one place to another until she finally winds up on the Carribbean island of Fear Cay.

Doc and his aids follow in pursuit, as does the mysterious Dan Thunden. Once on Fear Cay a battle ensues for the secret of eternal youth which lay hidden for centuries in the depths of a  huge network of underground of tunnels!