The Captain replaces Batman he was in Quinjet when Doomsday chased him and managed to destroy the Jet's tail and then lost control and then hit the ground. Cap survived but unfortunately Doomsday was there ready to blow him up again, at that time Captain Marvel made a badass entrance like Wonder Woman to save Steve's ass.
Fat Thor swapped places with Superman
So Fat Thor, Captain Marvel and Captain America vs Post-nuke Doomsday
No BFR, no Kryptonite
No prep or knowledge