The movie starts with the backstory of Infinity Stones and how powerful they are. It explains how a society of sorcerers was appointed to protect these stones from falling into wrong hands. Oops, wrong movie. The backstory is about the various astra which can harness the powers of various elements and animals. The most powerful of these astras is Brahmastra. Which for safekeeping was divided into three pieces and given to three different guardians for protection.
One of the guardians of these pieces is a scientist named Mohan Bhargava (SRK), who somehow keeps this very important thing on his work desk that he has set up on his balcony. (I would recommend you to watch this entire scene of goons confronting SRK in mute so that the silly dialogs do not distract you from the decent VFX work done here.) SRK also has a payal wearing that can make his channel the power of monkey, which can help him jump around a bit but it still is unable to make him fight these goons. He is then captured and tortured for information regarding the second piece of Brahmastra.
Before the second guardian is introduced we are told about the DJ Shiva, who sees a girl Isha in a party. Shiva somehow convinces Isha to join him for a birthday party. There Isha discovers that Shiva takes care of a few orphan kids and instantly falls in love with him. Then Shiva starts getting the vision of SRK being tortured by Junoon (Mouni Roy) and runs away leaving Isha alone.
Next day he goes to meet Isha and she agrees to go with this person on a life threatening adventure to save the second guardian of Brahmastra, who is an archaeologist played by Nagarjuna. Why does she decide to go with Shiva, because her name Isha means Parvati so she thinks she must follow Shiva everywhere. (If you had to keep this scene, it would have made more sense to name the hero Shiv, because Shiva also means Parvati. If only Isha knew a little better.) But maybe saving the life of Nagarjuna was not that important, they go on sightseeing singing and dancing, making love storiyan.
When they see Junoon they realise the actual reason they came to Varanasi and rush to find Nagarjuna. Here we are revealed that Nagarjuna has a Kangan that enables him to harness the power of Nandi. These astras are not very effective though (as clear from SRK cameo) Nagarjuna is shot and all three decide to run away to the secret ashram somewhere in Himalaya. Looks like they type “Guruji’s Secret Ashram” on google maps for directions.
On the way to the Ashram they are again confronted by Junoon and her two goons. Nagarjuna gives the piece of Brahmastra to Shiva, which he carries in his pocket wrapped in a newspaper like it was wada-paav and stays there to fight Junoon. Junoon invokes Kavacha-astra (really? Does no-one in team Brahmastra know the difference between a Kavacha and an Astra?) and is protected from Nandi-astra attack. But as established before these astras are useless, Nagarjuna also is easily killed by these goons.
So far only two characters have died and both of them possessed some powerful astras. Why exactly did we need a secret society to guard them? They are more suitable for circus tricks than fighting.
By the way Junoon wears a bajooband which gives her powers of some astra. There is no concept of being worthy or earning the right to use astras. Anyone wearing these jewellery can use these astras.
Eventually, Ranbir and Alia reach a closed gate but they are followed by one of the goons wearing the Vanar-astra payal. When he attacks Alia, Ranbir is able to invoke Agni-astra and burn the goon.
Enter sorcerer supreme, Amitabh Bacchan. Sorry, Guruji. He tells Ranbir that he is agni-astra. Then they send Alia to recover a conch-shell from Ranbir’s house which belonged to his mother. Then Guruji reveals that his father was Anakin Skywalker, who after acquiring the knowledge of force became Darthwader. Sorry, Dev.
Shiva’s mother helped defeat Dev and the third piece of Brahmastra is hidden in that conch-shell that Alia recovered.
I forgot to mention that there are other kids also in the Ashram who are guardians of different astras like Nag-astra, Vayu-astra etc. Guruji himself is guardian of Prabhastra (basically he can convert a knife into a blue sword made of CGI. I told you not more than circus tricks)
In the meantime Junoon goes to Dev, who is made of stone right now and asks for help. Dev gives him a piece of coal, which Junoon converts into many pendants and puts them on some villagers. These villagers become surveillance drones controlled by her.
Long story short Junoon ends up in the Ashram with her mind-controlled zombies and climactic CGI fight starts. Eventually Junoon is able to assemble all three pieces of the Brahmastra together and the power of Brahmastra starts destroying the world but then the world is saved by Prem-astra when Ranbir kisses Alia.
Us din Amithabh Bacchan ko ek baat pata chali ki Brahmastra se bhi shaktishali agar kuchh hai to wo hai prem. Aur hamein laga agar ye baat inko Mohabbatein ke time mein pata chal gayi hoti to hamein 3 ghanton ka torture nahin bardasht karna padta.