Several time-domain flooding simulation codes have been developed and improved over the past decade, after the previous international benchmark study in 2007. Consequently, within the ongoing EU Horizon 2020 project FLARE, a new benchmark study was organized. The first part of this study focuses on different fundamental flooding mechanisms, characteristic for progressive flooding in damaged passenger ships, including up- and down-flooding, as well as extensive horizontal flooding along a typical deck layout. Numerical results are carefully compared against measured water levels at different locations. Similarities and differences between the codes and applied modelling practices are discussed, and the reasons for observed deviations are analysed.