It is based on a book with a similar name (Fuglekrigen, or "The Bird-War" in English) from the writer, Bent Haller.

[2] In a forest lives a peaceful community of birds, including a pair of sparrows residing happily in a nest awaiting the hatching of their seven eggs.

The issuing struggle causes the seven eggs to fall out the nest with only one surviving and landing near a hole in the tree which is owned by Walter, an owl.

Betty goes to a nearby city and meets Olivia, an orphaned sparrow whose parents were also killed by Fagin, and adopts her as well.

Betty goes away again, and Oliver and Olivia wander off and befriend two orphaned mice named Fredrick and Ingolf.

They play, but Ingolf, who dreams of flying like a bird, falls into a stream and is saved by the group.

The following day, Fredrick and Ingolf decide to help Oliver with his battle towards Fagin with the trap being set up with his friends along with Armstrong.

Oliver's friends carry him into a hollow log where Olivia tells him that Betty was killed saving him and the plan didn't work, making him lose hope.

After nearly being attacked by a group of birds who've become fed up with their attempts to take down Fagin, Oliver gets angry with Olivia for teasing him to love her.

Fredrick and Ingolf now take care of themselves and Walter telling stories to Oliver and Olivia's children.