For those who are desperate for cash, drowning in vice and debt, and with no other means of attaining money lawfully, one is eligible for an absurd offer: a cash loan with an interest rate of 50 percent to be paid back in 10 days. That is the service Kaoru Ushijima, the "loan shark," offers.

In a society in which he believes one cannot live without either taking advantage of or being taken advantage of, Ushijima strives to be the former, operating in the black market and issuing loans to those he knows cannot afford the principal, let alone the mounting interest. However, he intends to have every yen he gives out to be paid back in full.

Yamikin Ushijima-kun follows Ushijima as he wrings his dues out of his customers, believing all of his clients to be undeserving of his compassion. Whether it be prostitution, identity theft, or extortion, there are no miseries Ushijima is unwilling to put his customers through as he journeys through the underworld.