A young man Kanan (Suraj Sharma) returns to India from Canada to marry his long-term girlfriend Anu (Mehreen Pirzada), but comes to know that as he is a manglik (born under an unlucky star) he has to get married to a tree before getting married to her. He very reluctantly marries the tree, which is duly chopped down after the completion of the ceremony. As a result, from that day onwards he is haunted by the spirit of a woman named Shashi (Anushka Sharma), who lived in that particular tree and hence claims to now be 'married' to him.
The story of Phillauri and Shashi is slowly revealed via the latter's flashbacks. Back in an unspecified time period, Shashi is a bright young woman who never fails to read the works of a poet named Phillauri that is regularly published in a local daily. Everyone in the village of Phillaur knows that the poems are written by the singer Roop Lal 'Phillauri' (Diljit Dosanjh), who is the heart-throb of the village. However Roop Lal notices that Shashi, unlike all the other village girls, never comes to hear him sing.
When he confronts her one day and tells her that he is the Phillauri who writes the poetry by narrating one of his famous poems to her, he is shocked when Shashi slaps and insults him. She asks him to use his talent – of being able to connect with the common man through his songs – for something important (especially when the struggle for independence is on) and not for frivolous matters. From that day onwards Roop Lal is a changed man, dedicating his life to knowing and understanding the poetry of Phillauri (which it is clear now that he is not the author of), and spreading its message via his beautiful singing to others.
On one particular night while Roop Lal is alone at home singing one of Phillauri's poems, Shashi comes to him and reveals that she is the one who writes poetry under the pseudonym of 'Phillauri' (which she can't do openly, being a woman). Subsequently love blossoms between the two (the poet and the singer who translates her poetry to song and spreads its message far and wide), but Shashi's elder brother (a reputed doctor of the village who brought her up) eventually comes to know about the affair and intervenes between the lovers. Roop Lal confronts Shashi's brother and admits to having been an alcohol-addicted good-for-nothing who still is not worthy of her, but tells him that he (Roop Lal) is going to Amritsar to record all the songs written by Shashi and will eventually return and ask for her hand in marriage.
The very fact that Shashi writes poetry is a revelation to her brother, and as time passes, he too reads the poems and starts appreciating her obvious talent. Meanwhile Roop Lal records the songs in Amritsar (the gramophone record is credited to Roop Lal 'Phillauri', thus being an amalgam of his name and her pseudonym) and is paid a handsome fee of three hundred rupees. He immediately sends the entire amount as a money order to Shashi's brother, along with a letter stating his intention to return and marry her on Baisakhi. Shashi's brother, by now convinced of Roop Lal's changed nature and good intentions as well as the deep love between the couple, starts the preparations for her marriage.
On the day of the wedding Shashi comes to know that she is pregnant, but does not disclose the news to anyone except her close friend Amrit (Nidhi Bisht). She expectantly waits for Roop Lal along with all her relatives and the entire village, but eventually the day ends with him nowhere to be seen on any of the buses from Amritsar. Her brother has to face the intense embarrassment and shame of sending everyone away, and this in turn hurts Shashi so much that she commits suicide (by hanging herself from a tree) due to an overwhelming sense of sadness, despair and betrayal. In the midst of all these flashbacks by Shashi's spirit the parallel storyline in the present continues with Kanan (whose confused ramblings about seeing a ghost naturally no-one believes), ending up in trouble with Anu, who feels that he is not really interested in marrying her (which is partially true since he is getting cold feet).
In order to clear the misunderstanding Shashi finally manages to reveal her presence to Anu as well (though Anu still cannot see her), and having heard her entire painful story Anu realises that Shashi's spirit is in fact still stuck on the mortal plane due to her unfulfilled love. At that point Anu's grandmother happens to play her favourite record, which as fate would have it is the one Roop Lal recorded. The record label mentions the recording date as 13 April 1919, which results in an epiphany for Kanan, who immediately takes Anu (with Shashi in tow) to the site of the Jallianwala Bagh massacre, which took place on that very day 98 years ago.
The movie ends with Shashi's spirit reuniting finally with her lover Roop Lal's (who was indeed killed in the massacre on the very Baisakhi day he recorded their songs and was to return to marry her). Before they both ascend to heaven Shashi tells Kanan and Anu (who can now see her) to love and cherish each other always, and that is what they promise each other as they embark on a new phase of their own sweet and endearing love story.