3-gatsu no Lion Rei Kiriyama is a 17-year-old boy who recently started living alone, financed by his salary as a professional Shogi player. Despite his independence, however, he's yet to mature emotionally, and his problems continue to haunt him in his daily life. His relationship with his adoptive family is strained, and he has difficulties interacting with his fellow high school students. Meanwhile, his professional career has entered a slump. Burdened with the heavy expectations placed on him as the fifth to become pro in middle school, his wins and losses are fluctuating as his record and progression into the ranks begin to stagnate. Acquainted with Rei are the three Kawamoto sisters: Akari, Hinata, and Momo. Unlike Rei, they live happily in their modest home, which they warmly welcome Rei into as if he were one of their own. Despite his reservations about becoming too close to the family, he frequently visits, interacting with them and receiving the kind of care and affection he never quite had while under his foster home. This is the story of Rei's triumphs and failures, relationships new and old, and his growth as a person.