Following the events of Infinity war the main Avengers left are Captain America, Thor, Hulk, Iron Man, Black Widow, Hawk Eye, Captain Marvel, Ant Man etc.
They decided to with help of Quantum Realm they will travel in past and bring all infinity stones to reborn the universe which is destroyed by Thanos with his swap.
They make three teams and going to search the Infinity Stones in year 2014. As they are searching the stones they encountered Thanos and its army again. But this time they are collecting all Infinity stones and made another glove to end the Thanos and his army and all evil stuff.
In this effort they lost Black Widow and lastly The Iron Man.
So half the universe is retain again. Hail Avengers