Ui confessed his love to Natsuno but was gently rejected to be as friends. Years later of being separated, they meet again at a wedding but Ui's injury apparently caused him to lose some of his memory, including Natsuno. When they met, Natsuno was made homeless because he broke with his girlfriend. Tired of staying at the company, he took his chances to persuade Ui into letting him stay. He lied that they were exes and pointed out that he could help Ui with his injury. Unknown to Natsuno, Ui only lost a month's worth of memories, but vividly remember how he was rejected by Natsuno. In provocarion, he lets Natsuno prove that they had a relationship with a kiss.
Well, the beans spilled when Natsuno realizes that it is better to tell the truth to which Ui countered with his truth as well. Long stroy short, the confession years ago becomes mutual.
Warui Otona ni Narimashite