Even though Karl Stern didn't look nor consider himself a Jew, it made no difference in Hitler's Nazi Germany. Karl meets a boxing champion through his father's connections. At the Berlin Boxing Club, Karl gains self confidence and physical strength. However, the only way he can continue training and competing is by keeping his Jewish heritage a secret. Karl's abilities become even more critical as anti-Semitic measures puts his and his family's lives in danger.
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Image from https://www.goodreads.com/book/photo/9266762-the-berlin-boxing-club