Episode 15 of "Friendly Rivalry" kicks off with Seul-gi jolting awake from a dream where her underwater bubble shatters, leaving her feeling precariously unbalanced. This disquiet translates into her performance during an examination, causing her to plummet to 20th place instead of her usual top spot. Concurrently, Je-yi finds herself ranked 15th, while Choi Gyeong triumphantly claims the number one position. Motivated by a desire to excel, Je-yi turns to her father, requesting challenging questions akin to those he had previously prepared for Je-na in preparation for college entrance exams.
Elsewhere, Seul-gi is disheartened to learn that her stepmother has resolved the case, a revelation that stirs her frustration to boiling point. In a fit of anger, she ventures to the columbarium, shatters the glass encasing Su-jin's ashes, and retrieves them. News of her actions reaches Tae-jun, who confronts her and subsequently strikes a deal: access to the final exam questions in exchange for her silence. Seul-gi responds with a sly grin.
Meanwhile, Byeong-jin and Ye-ri's collaboration continues unabated, with Byeong-jin discovering Choi Gyeong's examination triumph. Later, he stumbles upon her looking drowsy at a road crossing and "gifts" her a packet of pills, hinting at their potential benefits.
Unable to reach Je-yi directly, Seul-gi resorts to blackmailing Ara into aiding her scheme. She tracks Je-yi down at the swimming pool, her threat to plunge into the water a chilling ultimatum if Je-yi refuses to converse.
Realizing Tae-jun's steadfast commitment to "assisting" Seul-gi, Je-yi devises a plan to allow Seul-gi to directly procure the exam questions. Tae-jun concurs, and Seul-gi approaches the math teacher tasked with crafting the exam, employing blackmail—unbeknownst to her, a shadowy figure captures the interaction on camera.
Ye-ri's drug venture thrives, her clientele growing steadily. It is also disclosed that Choi Gyeong has been denied admission into her chosen university's medical program, a devastating blow.
At school, Ye-ri's astonishment grows when she discovers Choi Gyeong among her clientele. However, she refuses to sell her pills, believing it to be in Choi Gyeong's best interests. Their conversation veers into the territory of secret tutoring, a revelation Seul-gi overhears by chance. Yet, neither Ye-ri nor Choi Gyeong delves further into the matter.
Later on, Seul-gi's attention is drawn to wisps of silvery white hair clinging tenaciously to Choi Gyeong's socks, sparking her curiosity. This curious discovery propels her to Choi Gyeong's abode, where an unexpected sight greets her – Je-yi's beloved dog. Reflecting on Je-yi's habit of meticulously preparing for eventualities to cushion the emotional blow when they unfold, Seul-gi experiences a sudden, clarifying epiphany. With urgency, she dashes towards the swimming pool, her voice echoing as she frantically calls out Je-yi's name, the scene closing on a cliffhanger.
