There is a rich literature on students' and teachers' intuitions and misconceptions about probability. However, less attention has been paid to the development of pre-service teachers' probabilistic thinking in teacher education. Based on this, the second author developed a lesson sequence for teaching probability. In particular, it demonstrates how a game context can be used to explore the relationship between experimental and theoretical probabilities in a collaborative learning setting. The lesson sequence integrates concepts and processes related to probability and is grounded in socio-cultural theory. We trialed the sequence with secondary pre-service teachers. This paper focuses on their understanding of the probability concepts embedded in the sequence. Video and audio data indicates that while teachers used a range of strategies and data displays to explain the ideas integrated in the lessons, some reverted to equiprobability bias. The findings also reveal that pre-service teachers can modify their thinking when engaged in rich teaching and learning tasks.