Mazhai is a remake of the Telugu hit Varsham. Arjun (Jeyam Ravi), an unemployed youngster and Deva (Rahul Dev) a powerful don both fall in love with Sailaja (Shriya), a middle-class beauty, at the same time on a rainy day at a railway station. Arjun keeps bumping into Sailaja; coincidentally every time they meet, it is raining. This makes them both feel that it is perhaps the rain that keeps bringing them together, and they start falling in love. Deva, on the other hand, takes the help of her good-for-nothing father (Kalabhavan Mani) to get the girl.
After learning of the Arjun and Sailaja’s love, Deva and Sailaja’s father work up devious plans to break them up, finally succeeding in creating a rift between the two lovers. Though Arjun realises what is really happening, Sailaja falls prey to the foils of Deva and her father. The young lovers part ways; a film producer (Pyramid Natarajan) meets Shailaja and casts her as heroine in his film; Sailaja becomes a leading lady in movies while Arjun becomes a demolition expert with his uncle in a quarry.
When Sailaja becomes a film star, her father does not want Deva to marry her since he wants to keep for himself the goose that lays the golden eggs, and avoids Deva, cheating him of some money. The lovelorn and money-thwarted Deva, cheated by Sailaja’s father, finds out where Sailaja now lives and kidnaps her, right audaciously and thrillingly. Sailaja’s father now realizes that the only person who is daring enough to get his daughter back is Arjun and goes to him with his pleas. It takes much convincing, but Arjun finally decides to rescue Sailaja… but for his own reasons, reasons that he reveals later. But don't worry; this is a typical Tamil movie, and the end is conventional - they make up, her father repents, and they live happily ever after.