The opera is set in India in the 19th century. As the action begins, two British officers, Gerald and Frederic, are walking with some friends near the home of Nilakantha, a Brahmin priest, with a temple nearby. Nilakantha is looking for vengeance against the occupying British, who have forbidden him from practicing his religion. Before long, Nilakantha quietly goes off for a secret meeting with some of his Hindu followers, leaving his daughter Lakmé behind with her servant Mallika.
The two young women go off toward a river to gather flowers and bathe, and sing the famous "Flower Duet." As they approach the water, Lakmé removes her jewelry and leaves it on a bench.
Gerald and Frederic then approach, with their friends Ellen and Rose, looking for a place to have a picnic. They've broken through the fence surrounding the temple, and Ellen catches sight of the jewelry. She admires the beautiful designs, and says she'd like to have drawings of the pieces to take with her, and perhaps have copied. Gerald is the artistic one in the group. He agrees to stay behind and make some sketches as the others move off.
When Lakmé and Mallika then return Gerald is startled and hides. Mallika heads for home, leaving Lakmé alone. When Lakmé spots Gerald, she's frightened. She cries out, and people come to help. But Lakmé is soon intrigued by Gerald, and sends them away. There's an immediate attraction between Gerald and Lakmé. But she knows they can never be together, and tells Gerald to forget he ever saw her. Then Nilakantha returns. He's furious that Gerald has trespassed on the sacred grounds of the temple, and he swears to get even.
ACT TWO begins in a crowded and colorful marketplace. Nilakantha makes his way among a crowd of Indians and English soldiers, with a plan in mind. He asks Lakmé to sing, knowing that if Gerald is nearby, he'll be drawn to her voice. What we hear is the well-known aria called the "Bell Song," with its tinkling, high coloratura.
The song does the trick, and Gerald soon appears. But when Lakmé sees him, she stops singing, and faints. Gerald rushes forward to help her, but Nilakantha steps out of hiding and stabs him. Gerald is only slightly injured, and Nilakantha flees. Lakmé and a servant, Hadji, take Gerald to a safe hiding place.
ACT THREE opens in the forest where Gerald is recovering. As Hadji watches over him, Lakmé arrives. There's a sound of singing, far in the distance. Lakmé tells Gerald it's a band of lovers going to drink from a sacred spring whose waters will bind them together forever. Lakmé determines she and Gerald should also drink the sacred water. But when she leaves to fetch some, Gerald's friend Frederic turns up. He reminds Gerald of his duty to England -- and that they'll soon be assigned to a new post, far from India. Gerald knows he'll have no choice but to follow orders, and abandon Lakmé.
When Lakmé returns, she senses that Gerald's mood has changed, and seems to know that he'll be leaving her. In desperation, she picks a poisonous leaf from a nearby tree, and eats it. Realizing what she's done, Gerald is overwhelmed by her devotion to him. He takes the cup Lakmé has brought from the spring, and they both drink the sacred water -- a holy declaration of love and fidelity that even Nilakantha is powerless to revoke. Lakmé dies in Gerald's arms, as her father looks on, and the opera ends.