So here's how I think the movie plays out given the trailer. The movie will open on Nova Prime, home of the power stone. The Black Order will make short work of the Nova Corp and gain possession of the power stone and the scene will end with Thanos making his first appearance to claim the stone from the Black Order. Roll credits.
The next scene of the movie will center on the Asgardian ship with Thor and Loki. The Black Order will capture the ship and kill the crew. Thor and Loki will work together to try to save as many of the Asgardians as possible, and Loki will hand over the Space stone as part of a gambit to negotiate with Loki. In the process of trying to save the Asgardians Thor will be propelled into space and the Hulk will somehow be propelled to Earth.
The Hulk crashes into Dr. Strange's sanctum where he reverts to Banner and plays the part the Silver Surfer played in the Infinity Gauntlet story: he will herald the coming of Thanos. This brings Tony Stark to the sanctum to hook up with Banner. By this time the Black Order descends on New York where Spiderman senses the danger. Tony calls Steve Rogers on the flip phone Rogers left for him (I think that's the phone in the trailer) and asks Strange to send Banner to Wakanda. Stark, Strange, Wong, and Spiderman fight the Black Order in New York. With Thanos in possession of the Power and Space stone they are no match for him, but Thanos doesn't want to simply obliterate them, he wants them to see him win all the stones and witness what he will do with them once he has the full gauntlet assembled. It is unclear if he acquires the Time stone or if Strange somehow manages to keep it out of Thanos' hands.
The Black Order also attack Vision who is living with Scarlet Witch to try to take the Mind stone. Rogers appears and fights them, but its unclear if they take the stone or not. I think it is possible they take the stone but the Vision survives.
In Wakanda Banner works on the Hulkbuster armor. He doesn't want to turn into the Hulk as he is still afraid if he turns into the Hulk the Hulk will take over as he did in Sakaar, and he figures by upgrading the Hulkbuster armor he can in effect be the Hulk without having to turn into the Hulk and still be useful in a fight. Wakanda becomes the last stand for the Avengers where Thanos' army and the Black Order try to take the Soul stone from Wakanda, and Thanos with the infinity gauntlet takes on most of the Avengers.
I have a suspicion that the reality stone is not going to appear in Infinity War: it will make its appearance in the second movie. The second movie will be more of a space/interdimensional fight where Dr. Strange, the Guardians, Thor, Ant Man, and Captain Marvel will feature prominently. And I think Loki will be central to how Thanos is eventually defeated.
That's what I got from the trailer so far.