Mareta Percival [Anapela Polataivao] is a Samoan New Zealander who works as a teacher in a primary school in the low‑income Christchurch suburb of Aranui. On the day of the 2011 Christchurch earthquake, Mareta's daughter auditions at the CTV Building and is killed during the quake. A grief‑stricken Mareta loses her sense of purpose in life while clinging to her Roman Catholic faith.
In 2014, to maintain her unemployment benefits, her nephew and social worker Sio, [Beulah Koale], convinces Mareta to apply for a job as a substitute teacher at St Francis School, a largely Pākehā and rigidly conservative private school. Despite reservations from the school board, the retiring headmaster Alan Hubbard, [Dalip Sondhi], convinces them to hire Mareta as a substitute. After walking in on a student, Sophie [Antonia Robinson] being confronted by deputy principal Peter Wadsworth [Jamie Irvine] to specialize in an instrument, Mareta covers for her by pretending they had discussed vocal scholarships. Since St Francis lacks a choir, Mareta convinces Alan to allow her to start one with herself as its choirmaster, much to the disdain of Peter, who tries to undermine Mareta and the choir. Mareta also enlists the help of St Francis alumni and accomplished chorister Helen, [Tania Nolan].
The choir attracts several students other than Sophie, such as Anthony Bull [Zac O'Meagher], Mei‑Ling [Talia Pua], and Luke. Despite their privileged upbringing, these students—particularly Sophie—struggle with personal traumas and insecurities. As a choir, the students learn to work as a team. Seeing potential in Sophie, Mareta trains her as lead chorister. With the encouragement of her Catholic priest Father McAfee, [Matthew Chamberlain], the St Francis choir conducts their first performance at Mareta's Samoan church. After the performance, Mareta's former teaching colleague, Rona, [Nicole Whippy], confronts Mareta as she views St Francis kids as privileged and elite. This leads Mareta to pass out and be hospitalised, revealing her cancer diagnosis which she had hidden.
Mareta introduces her students to the upcoming national Big Sing competition hoping to get them to compete in it, which her students agree to. However, due to a troubled home life, Sophie leaves the choir after refusing to don a lavalava due to a disfiguring arm injury, forcing Mei‑Ling to take a leadership role. Tensions flare up after Anthony and Mei‑Ling are embroiled in a fight with St Francis' elite rugby team, which Anthony is a part of, leading to Anthony also leaving the choir.
Despite these obstacles and her own terminal cancer condition, Mareta is determined to lead the choir to perform at the Big Sing competition. However, Peter uses the fight between the choir and rugby team as reason and convinces the school board to pressure Mareta into ending the choir programme. Due to the students' support, defying Peter, the St Francis choir participates in the semi‑final and qualifies for the final of the Big Sing. However, as a result, Mareta's teaching contract is terminated.
Mareta is hospitalised due to her worsening cancer. However, the choristers are still determined to participate in the final round of the Big Sing. Sophie and Anthony return to the St Francis choir. In order to compensate for Mareta's absence, Sophie enlists the help of her mother and board member Caroline, [Alison Bruce], and Rona's support. On the night of the competition, Mareta succumbs to her cancerous condition, but, after doubts and anxiety leading her to almost leaving, Sophie leads the St Francis choir, wearing a lavalava, to perform a traditional Samoan song for the final round.
