Disputed property

Dear Sirs, My father was an state government employee, some time in 1980's time, state government allotted open plots for MLA's MP's & State government employees. They have formed Housing society and they have released notice that if any one like to buy the plots can come to housing society and register their names by paying society fees of Rs 110/- and allotment cost of Rs 12000/-. After few years my father came to know that plot has been alloted for 400 sq yards. After some more years one of the representative of the housing society came to my father and asked to sell the plot and my father agreed on terms of at that time of market value rate & the society representative agreed. The representative came to my father and paid Rs 12000/- which was paid at the time of allotment of the plot and the representative told my father that the remaining amount will be paid to us very shortly but not paid. Again after few years the officials of Anti Corruption Bureau came to my father enquired about our plot . My father given the statement and expalin the whole thing what has happened Very fortunate thing is my father never done any signature on any paper. Again ACB Officials came to my father and explained that the housing society has been cheated. The housing society secretary is an normal clerk of state government he earned lakhs and crores of rupees on these plots sold to other parties by doing forgery signatures. ACB Officials sent an summons to my father to appear before court for witness . My father appeared infornt of Honourable judge and explained whole thing and came back. After that we went to sub registrar office and checked our Encumbrance certificate and we are surprised that still the property is existing on my fathers name. When we went to check our plot we are surprised that a nice posh duplex house with swimming pool and lawn is established on our plot. Our plot area is around 400 sqyards the present value of the plot is 4 crores. Now, we would like to file a suit . please suggest how to proceed further legally.