The Avengers have found a lightly guarded time-portal in the basement of a Japanese Hydra base.
(Now that's a sentence that truly drips with the fragrant odor of Marvel Comics).
Fortunately, the time portal controller runs on a "Starkware" operating system, so Iron Man is able to take control of it.
(Oh dear! What a tragic oversight for Hydra to make. How embarrassing!)
The portal is apparently configured to connect to two different time-zones: Mid 20th century (i.e. WWII) and about 10,000 B.C. Captain America decides the smart thing to do is to split up.
(Note: That is NOT the smart thing. You don't NEED to split up when dealing with time travel... because you HAVE all the time you need. You can just go to both time zones one after the other, and you won't ever be late!)
Then, despite deciding to split up... all the heroes jump into the time portal at once... and travel together. But through the magic of plot devices, the teams do get split up. Into three groups:
There's a small Hydra squad in the past, living in a jungle among the Japanese cavemen. The Hydra guys are using mind-controlled giant animals (like Lions and Wooly Mammoths and other creatures that don't belong in pre-historic Japan) to guard a huge empty hole in the ground. Why giant animals? Why guarding an empty hole? Because it fills up the page count, silly!
Cap, Barton, and Pietro quickly defeat the villains, and then learn that thanks to "a distant future Hydra station projecting temporal static," it's not possible for anybody to come through the time portal back to this time and rescue them.
(Really? Seems to me that regardless of "temporal static", the three heroes just arrived pretty accurately to meet the Hydra team right on schedule. But apparently that was just "random", according to the Hydra guy.)
Anyhow, it doesn't matter. Cap has a plan to get out of this mess.
It's really is quite impressive.
I mean, it's impressive how this story manages to have so much going on, yet with so little actually happening.
I just can't find anything to care about. There's global destruction, time-machines, special effects, cavemen, mind-controlled wild animals, brutal but cowardly Hydra thugs, Japanese warships, and kitchen sinks. In the middle of it all, Spider-Man and Hawkeye are childish and annoying, and everybody else just seems bored.
Oh, yeah. The dinosaur on the front cover doesn't actually appear. There is dinosaur in the comic in the comic -- a flashback to a battle in Manhattan "last week".
But honestly, even if there HAD been a dinosaur in the comic, it wouldn't have helped.