In a prison cell on Skaro, the Doctor and Jamie watch as Maxtible is paid by the Daleks for services rendered.
"The secret you promised me!" he demands.
Maxtible is a wealthy, propertied, Victorian gentleman who thinks of everything in terms of business transactions. His charity to Waterfield and his daughter has given him - Maxtible - proprietory rights to use them as he wishes, as servant and collateral. His pact with the Daleks is a "partnership". He is providing a service in return for payment. His payment is to be a secret that he has been pursuing fanatically, at the expense of anyone who gets in his way.
"The secret of transmuted metal," confirms the Black Dalek.
A series of formulae flash up on a screen. For all the talk of "atomic weight" and "specific gravity", the details are occult. But the transmutation is achieved.
"Gold!" cries Maxtible, "Iron into gold! I told you it was possible! They've kept their promise! It's true, it's true! They have!"
He thinks he has completed the alchemist's project... but that project was about harnessing purity and immutability in the hope of returning humanity to its nature before the fall. In aiding the Daleks, Maxtible has done the reverse of this: he has helped harness 'the human factor' in the service of utterly fallen things.
In any case, his lust for gold is nothing to do with purity. It is the general obsession of any Victorian gentleman. It's the basis of Victorian society. The secret he obsesses over is the secret of capital. The seemingly magical ability of capitalist production to create wealth from nowhere, from commodities that come out worth more than the raw materials that went into them. From dross comes gold - straight into the gentleman's pockets.
This is the basis of all Victorian gothic: the occult and occluded nature of profit, of capital accumulation.
The real secret, the thing occluded, is 'the human factor'. Human work, or 'species-being', alienated from humans and appearing to Maxtible in the form of Daleks, in the form of alien machines that drain humanity and substitute their own essence... which is what happens to him immediately after he is paid for his services.
His real payment is to be given 'the Dalek factor'. To be made into an occult, gothic thing. A Victorian zombie.