Printed in Smash #91-#97, October 28 - December 9, 1967.
The Man from B.U.N.G.L.E. is enjoying a fortnight vacation fishing when he spots a mermaid basking nearby. Approaching her he falls into a trap and learns to his dismay that the mermaid was just a disguise for "that diabolical dame, Tania the Terrible". That woman has plans to blow up a dam for very bad reasons.