Pay attention to the contract clauses.

Sujong, who has been facing financial difficulties while raising his daughter on his own, receives an offer for a part-time position as a test subject for a study led by his university junior, Woomin. Lured by the attractive compensation, Sujong hastily signs the contract without thoroughly understanding the nature of the research, only to come to the shocking realization later that his body has been entirely transformed. "You suit being the one who gets taken better, senior." A mix of shame and pleasure ensues, culminating in a contract with no means of escape.