Assume that a man, Bhairav Surve, practising black magic in Dhamangaon village of Maharashtra, murders a child of his locality on 20th December 2012. Later, in December 2013, the legislature of Maharashtra passes the Maharashtra Prevention and Eradication of Human Sacrifice and other Inhuman, Evil and Aghori Practices and Black Magic Act, 2013 and by virtue of provision against Ex post facto law, Bhairav Surve can’t be prosecuted and charged under the mentioned Act as the commission of offence dates back to when the act didn’t exist.
The Landmark judgement governing this doctrine came in the year 1953, in case of Kedar Nath v. State of West Bengal. In this case, the Hon’ble Supreme Court of India observed that, whenever an act is declared as a criminal offence and/or provides penalty for same by the legislature, it is always prospective in nature and can’t be implemented retrospectively to uphold what is being said under Article 20 (1).
Dealing with a similar situation, in the case of Mohan Lal v. State of Rajasthan (AIR 2015 SC 2098) which involved Narcotics, Drugs and Psychotropic Substances Act, the court opined that, only conviction and/or punishments under an ex post facto law is prohibited under Article 20 and not the trial or prosecution itself.
However, an exception also exists to the restriction under this provision. In the case of Rattan Lal v. the State of Punjab, the Hon’ble Supreme Court allowed for such retrospective implementation of Criminal Laws, where the issue pertinent is, reduction of punishment in the said offence.