Hektoria Glacier on the Eastern Antarctic Peninsula underwent a rapid tidewater-style glacier retreat from 2022 to 2023 after the loss of decade-old fast ice in the Larsen B embayment. The glacier retreated 25 km between February 2022 and March 2023, of which at least ~9 km was lightly grounded ice. As of March 2023, the terminus has been stable, however dynamic thinning and acceleration are still occurring. Remote sensing data in the months following the fast ice break-out reveals an ice flow speed increase of up to 5-fold (300 m/yr to 1700 m/yr), and rapid elevation loss with a thinning rate increasing by 40-fold (2 m/yr to 80 m/yr). Hektoria and Green Glaciers underwent three phases of retreat displaying differing calving styles. During the first two months after the loss of the fast ice in January 2022 the Hektoria-Green ice tongue calved large tabular bergs. In March 2022, an abrupt change in Hektoria’s calving style was observed, changing from large tabular icebergs to buoyantly rotated smaller bergs, indicative of a transition to grounded ice. Following this transition, in November and December 2022 Hektoria underwent several short periods of rapid retreat, losing ~9 km of grounded ice in two months. These retreat rates for grounded tidewater ice are greater than any reported in the modern glaciological record. Here we examine the evidence for locating the pre-fast ice break-out grounding zone as well as the drivers that could cause such a rapid retreat. We propose the cause of the rapid retreat was due to the presence of an ice plain in the lower section of the glacier just upstream of the floating ice tongue, a region that was close to flotation prior to the loss of the fast-ice.