Like probably everyone else in the movie-watching world, I had my doubts about this. But I admit I was surprised at how well this worked for me.
Jojo is a 10-year-old boy. Unfortunately, he lives in 1944-5 Germany, and he's fully invested in the whole Nazi thing. To the extent that his imaginary friend is Adolf Hitler. He's off to Hitler Youth camp, but to the despair of the camp counselor and most of his fellow young Nazis, he still retains a few shreds of decency (being unable to strangle a bunny on command) and is extraordinarily inept at soldiering. That last bit leads to a near-fatal unfortunate grenade incident. Which (in turn) sends him, limping and scarred, back home to Mom (Scarlett Johansson).
Where he discovers that Mom is hiding a young Jewish girl.
None of this sounds remotely as if it could be funny. But it is ("at least for me") because imaginary-friend Hitler is a boob, and everyone else realizes the absurdity in all the mayhem.
The movie was nominated for six Oscars (including Best Picture) and won one ("Best Adapted Screenplay"). Scarlett Johansson was nominated for Best Supporting Actress, and I think Sam Rockwell was robbed by not being nominated for his role as Captain Klenzendorf, a cynical German soldier relegated to teaching Hitler Youth.