The Hindus go to perform their rites in a sacred Brahmin temple under the high priest, Nilakantha. Nilakantha's daughter Lakmé (which derives from the Sanskrit Lakshmi) and her servant Mallika are left behind and go down to the river to gather flowers where they sing the famous "Flower Duet." As they approach the water at the river bank, Lakmé removes her jewellery and places it on a bench. A party of British officers, Frederic and Gérald, arrive nearby while on a picnic with two British girls and their governess. The British girls see the jewellery and request sketches: Gérald volunteers to stay and make sketches of the jewellery. He sees Lakmé and Mallika returning and hides. Mallika leaves Lakmé for a while; while alone Lakmé sees Gérald and, frightened by the foreigner's incursion, cries out for help. However, simultaneously, she is intrigued and so she sends away those who had responded to her call for help when they come to her rescue. Lakmé and Gérald begin to fall in love with each other. Nilakantha returns and learns of the British officer's trespassing and vows revenge on him for his affront to Lakmé's honour.
At a bazaar, Nilakantha forces Lakmé to sing (Bell Song) in order to lure the trespasser into identifying himself. When Gérald steps forward, Lakmé faints, thus giving him away. Nilakantha stabs Gérald, wounding him. Lakmé takes Gérald to a secret hideout in the forest where she nurses him back to health.
While Lakmé fetches sacred water that will confirm the vows of the lovers, Fréderic, a fellow British officer, appears before Gérald and reminds him of his duty to his regiment. After Lakmé returns, she senses the change in Gérald and realises that she has lost him. She dies with honour, rather than live with dishonour, killing herself by eating the poisonous datura leaf.