I will present the science case and payload of WALTzER (Wide-band Atmospheric Laboratory for Transiting Exoplanet Research), which is an ESA F-class mission concept recently submitted to ESA's F3 call. The mission is designed to characterise the atmospheres of planets outside our Solar System. Its primary goal is to study the composition, vertical structure, and evolution of exoplanet atmospheres by analysing them from their lower layers up to the part escaping to space. The payload uses a single 35 cm telescope to simultaneously direct light into three instrument channels: a near-ultraviolet spectrograph, a visible spectrograph, and a near-infrared photometer. This design is strategically important, as WALTzER will fill the critical observation gap in the UV spectrum that is anticipated after the Hubble Space Telescope is decommissioned, and it would be the first instrument covering simultaneously from the near-ultraviolet to the near-infrared. The mission is a partnership between ESA and a consortium of ten member states, which is co-led by Austria and Switzerland. WALTzER’s data will directly complement observations from the JWST and Ariel missions by providing simultaneous near-ultraviolet and visible context to their results based on infrared observations. During its three-year nominal mission, WALTzER will function as a versatile observatory: in addition to surveying almost 100 exoplanets, it will support a broad range of scientific investigations in Solar System science and Galactic Astrophysics. To ensure wide scientific benefit, 25% of the available observing time will be allocated to the global community through open calls.