Desperate Mrs. Seon-ju

By any chance... are you four going together now? (1x41)

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Jin-ju questions Seon-ju about her relationship with So-woo, and Seon-ju accidentally reveals So-woo's relationship with Sang-ah. When Sang-ah finds out Soon-ae isn't her birth mother, she feels betrayed and starts suspecting Soon-ae's motives for raising her. Sang-ah decides to get Seon-ju out of her sight.