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After former prince, Lee Minho stepped out of line of the throne the royal family frantically tried to get a new heir. Finally after many attempts, prince Kim Seungmin was born.
To avoid another Minho Incident the new prince would never be let out, under no circumstances can the boy socialise. To ensure this, the king gives seungmin bodyguards to scare anyone who tries away and keep the prince safe.
Maybe not the best idea to have the very social Christopher Bang guard the prince, but that's fine.