The iconic Great Barrier Reef (GBR) experienced a mass coral bleaching event in early 2024. In the southern GBR, heat stress triggered severe and widespread bleaching to levels not previously recorded, affecting a diverse range of coral genera on One Tree Reef (OTR). We monitored the health of 462 coral colonies over 161 days from the peak of the heatwave to the autumn and winter cooling. 66 % and 80 % colonies were bleached in February and April. By May, 44 % bleached colonies were dead, and 53 % in July. In July, 31 % colonies were still bleached and 16 % had recovered. At Goniopora black streak disease has developed, contributing to high mortality. In Acropora Colony collapse occurred (95% mortality) with accumulation of algal-contaminated fragments. Monitoring of individual colonies in water showed rapid bleaching, disease onset, and mortality. The sheltered condition and offshore location did not protect OTR from bleaching and mortality due to heat stress. (More on aslopubs.onlinelibrary.wiley.com)