Old Hill Monastery in Marlow, a special prison for captured spies, is in trouble. Dangerous inmates,
such as Rostov, are literally disappearing from their cells. Steed is drawn into this case when his
friend, George Neville, is shot by Rostov after a party at Steed's flat. Steed knows that Rostov
was one of three enemy agents detailed to kill three top British agents. Neville was Rostov's
target, and Paul Ryder (another friend of Steed's) is Lubin's target. The third prisoner, Martin
Ezdorf, has refused to confess who his target is.
Ezdorf claims that Lubin will escape, as will he himself, to complete their mission. As Steed talks to
Ezdorf, Lubin does indeed escape and manages to disappear despite being trapped in an apparent
Drink too much and it'll turn you... invisible! All the better
for sneaking in from the pub without the wife noticing...
Steed asks Mrs. Peel to warn Ryder of the danger, but she arrives too late - only having time to see
footprints from Ryders spilt paint tin vanish outside the house. Steed goes back to Old Hill to
question Ezdorf, but the only thing he finds out is that Ezdorf's target is Steed himself. Mrs.
Peel is examining the prisoners' files and the contents of Lubin's cell. All three agents had a
piece of paper with CXVIIIVIXXV written on it. Obviously a roman numeral, she also sees that a
magazine from Lubin's cell, Bryant's Natural History Magazine, is issue number CXVIIIVI and that
page XXV (25) is missing. She goes to the magazine offices to find out what the missing page is,
but the publisher Cedric Bryant has been killed by Lubin. However, she does find a copy of the
page - which is about lizards - and she and Steed link the page to bottles of vodka from the
men's cells; the brand is Lizard Vodka.
The vodka is imported by the Magnus Importing Company, whose van Mrs. Peel had noticed at Bryant's
office. Steed visits the company, where he sees vodka destined for Ezdorf. He goes back to Ezdorf,
who taunts Steed that he will escape and kill him. Mrs. Peel takes the files back to the Ministry
where, seeing some pet chameleons, she gets a clue as to how the prisoners are escaping. After
Steed has left, Ezdorf's cell is checked by Colonel James and his men - it appears to be empty.
They check the cell but are surprised by an invisible Ezdorf who knocks them out and escapes. Back
in his flat Steed notices an advert for Lizard Vodka in the magazine, which causes him to discover
that the vodka bottle can be separated into two compartments. He goes to the Magnus Importing
warehouse, to discover what was in the bottles. Magnus tries to jump him but Steed knocks him out,
then manages to see that the mysterious liquid in the bottle can be used to make the user merge
with his background - like a chameleon. Whilst Steed is out, Ezdorf goes to Steed's flat. There
he captures Mrs. Peel, who has come to warn Steed of Ezdorf's escape, and the pair wait for Steed
to return. Very soon Steed returns, but neither Ezdorf or Mrs. Peel can see him; he is wearing
the coat Lubin used to escape. He knocks Ezdorf unconscious, freeing Mrs. Peel and putting an
end to Ezdorf's plan.