The story revolves around a special piano that holds a precious secret. Many years ago, a man by the name of Arumugam receives a wooden box as a gift from an anonymous Englishmen for his kindness. Upon inspecting the wooden box further in his house, Arumugam discovered a precious pink diamond, with no intention of selling the diamond, he and his piano-maker friend designed and built a custom-made piano that has a secret compartment that can only be unlocked by playing a unique tune. Arumugam splits the tune into four and teaches his four children separately. Years passed and Arumugam develops Alzheimer's disease, forgetting about the piano's secret, neither does his family had any knowledge of it but things started to go for the worse when several dangerous and mysterious figures finds out the piano's secret and goes after Arumugam's family for the diamond.

Moondram Athigharam tells the story of how a sturdy police officer named Ashok and a brainy forensic expert, Kalki team up to bring down a serial killer named 'Ruthran' who challenges them both.

Aiyavu (Lingeshvaran Maniam) and Mani (Kuben Mahadevan) are two small-time gangsters working as henchmen for Veera Anna (Senthil Kumaran Muniandy) who handles the organized crime around the city and runs the business for Periya Anna (Haridhass S). Veera Anna entrusted Aiyavu to dispose of a gun, but the gun goes missing from his possession after a drunk night. Aiyavu & Mani searches for the gun before Veera Anna finds out about it!