Teaching "out-of-field" is a present obstacle in mathematics education in many countries, and developing professional learning programs aimed at upskilling non-specialist teachers is urgent. A teacher study group was established wherein two non-specialist teachers of mathematics (Years 7-10; aged 12-16) engaged with algebra to develop a deeper understanding of the subject and its teaching. The study group lasted one school year, during which the teachers actively participated in the learning. Multiple data sources were collected, including reflection forms, open-ended questionnaires, and a storytelling form. Analysis of the teachers' solutions to a sample of algebra problems and self-reports suggests that the study group enabled teachers to acquire new knowledge/skills in mathematics pedagogy, gain a new understanding of how student thinking and understanding develop, adapt new classroom instruction strategies, develop ability/confidence in problem-solving, and develop new knowledge/ability in algebra. The study findings contribute to our understanding of supporting non-specialist mathematics teachers' professional development.