It was somthing to do with drugs opium I think. Great Britian was given a 99 year lease on Hong Kong as some sort of penalty for China,s export of opium.
In 1898, GB obtained a lease from the Chinese government after they have won a war with China known as the "Opium Wars" under the instigation of Businessman from Great Britain who wants to export opium (DRUGS) into China but was ban by the Chinese Government. It was not a day of Glory for the British Empire. In fact, it was a shameful act by the British Government.
The answers given are correct but it should be noted that only parts of Hong Kong were ere leased, specifically the New Territories and outlying islands. Everything on Kowloon (the mainland) south of Boundary Street and Hong Kong Island was never leased. It was just logistically impossible to give back only the leased bits of the country.