Set in the azure and sultry backdrop of Goa, Zeher tells the tale of a young cop, Siddharth (Emran Hashmi), who is on the verge of a divorce with his wife, Sonia (Shamita Shetty).
His marriage almost over, Siddharth finds himself drawn towards a local married woman named Anna (Udita Goswami), who is stuck in a bad marriage with a man who beats and abuses her. She is also suffering from cancer. Though her husband knows that, he doesn't care about her.
Unable to cope with the pressures of his wife's success, who is in the special police force, Siddharth continues his secret and dangerous relationship with Anna.
Things get even more complicated when Siddharth takes an irrational decision to give Anna the money he recovered in a drug raid, for her medication in one final effort to save her from dying of cancer.
To his misfortune, Anna dies in a bomb blast that very night. Siddharth now races to uncover a murky tail of drug money, murder and deceit...because all the evidences point to him.
The case of Anna?s murder is given to Sonia. With his wife heading the case, Siddharth is now in a race against time to find out the real truth behind Anna's murder, recover the drug money and also win his wife's love back.